Atmospheric pressure ion lens for generating a larger and more stable ion flux

ABSTRACT

An ion lens is used to focus ions produced by various types of ion sources which are substantially at atmospheric pressure. The ions are focused to the inlet of a downstream mass spectrometer or other devices which require a larger and more stable ion flux improved performance. The ion lens is mounted in close proximity to the sprayer tip. The ion lens increases the total ion count rate summed over all of the generated ions. The ion lens may also be employed to vary the degree of ion fragmentation and the charge state pattern of the generated ions. The ion lens may also result in a more stable ion signal. Furthermore, more than one ion lens may be used. This invention may also be extended to multisprayer ion sources.

REFERENCE TO RELATED APPLICATION

This application is a national phase entry application of PCTapplication Ser. No. PCT/CA01/00728 filed on May 22, 2001. Accordingly,this application also claims priority from U.S. Provisional PatentApplication Ser. No. 60/205,549 filed on May 22, 2000 and U.S.Provisional Patent Application Ser. No. 60/229,321 filed on Sep. 1,2000.

FIELD OF THE INVENTION

The present invention relates to various types of ion sources such as,but not limited to, ionspray, electrospray, reduced liquid flow-rateelectrospray, reduced liquid flow-rate ionspray, nanospray andatmospheric pressure chemical ionization (APCI) sources. Moreparticularly, the present invention relates to increasing the ion signalstability and the ion flux generated by various types of electrosprayion sources.

BACKGROUND OF THE INVENTION

Electrospray ionization (ESI) is a method of generating ions in the gasphase at relatively high pressure. ESI was first proposed as a source ofions for mass analysis by Dole et al. (Dole, M.; Mach, L. L.; Hines, R.L.; Mobley, R. C.; Ferguson, L. P.; Alice, M. B. J. Chem. Phys. 1968,49, 2240-2249). The work of Fenn and coworkers (Yamashita, M.; Fenn, J.D. J. Phys. Chem. 1984, 88, 4451-4459; Yamashita, M.; Fenn, J. D. J.Phys. Chem. 1984, 88, 4671-4675; Whitehouse, C. M.; Dreyer, R. N.;Yamashita, M.; Fenn, J. B. Anal. Chem. 1985, 57, 675-679) helped todemonstrate its potential for mass spectrometry. Since then, ESI hasbecome one of the most commonly used types of ionization techniques dueto its versatility, ease of use, and effectiveness for largebiomolecules.

ESI involves passing a liquid sample through a capillary which ismaintained at a high electric potential. Droplets from the liquid samplebecome charged and an electrophoretic type of charge separation occurs.In positive ion mode ESI, positive ions migrate downstream towards themeniscus of a droplet which forms at the tip of a capillary. Negativeions are attracted towards the capillary and this results in chargeenrichment in the growing droplet. Subsequent fissions or evaporation ofthe charged droplet result in the formation of single solvated gas phaseions (Kebarle, P.; Tang, L. Analytical Chemistry, 1993, 65, 972A-986A).These ions are then usually transmitted to a downstream aperture of ananalysis device such as a quadrupole mass spectrometer, a time of flightmass spectrometer, an ion trap mass spectrometer, an ion cyclotronresonance mass analyzer or the like.

Ionspray is a form of ESI in which a nebulizer gas flow is used topromote an increase in droplet fission. The nebulizer gas aids in thebreak-up of droplets formed at the capillary tip. Ions formed in thismanner can be directed into the vacuum system of various mass analyzerswhich include, but are not limited to, quadrupoles, time of flight, iontraps and ion cyclotron resonance mass analyzers.

Unfortunately, the use of ESI and ionspray with mass spectrometersresults in poor ion sampling efficiency. Typically, the majority of ionlosses occur between the atmospheric pressure region, where the ions aregenerated, and the first differentially pumped vacuum stage that theions must enter. Ions are formed in a broad plume of the electrospray,typically up to 1 cm in diameter. The ion sampling orifice, i.e. inletorifice of the mass spectrometer, is typically about 0.01 to 0.025 cm indiameter, and so only a small fraction of the ions pass through thesampling aperture. The size of the aperture separating the atmosphericpressure region from the first vacuum stage provides a conductance limitfor the flow of gas and ions into the mass spectrometer. The diameter ofthe aperture is limited by the pumping speed of the vacuum system of themass spectrometer. Due to the substantial expense associated with vacuumpumps, a compromise must be reached between the desired aperture sizeand the cost of the vacuum pumps. In addition, since the ion motion atatmospheric pressure is dependent upon the shape and distribution of theequipotential lines, many ions are not directed to the inlet aperture.

Accordingly, there have been attempts to increase the ion samplingefficiency which have led to the development of nanoelectrosprayionization (Wilm, M.; Mann, M. Anal. Chem. 1996, 68, 1-8) and otherreduced flow rate electrospray ionization sources (Figeys, D.;Aebersold, R. Electrophoresis, 18, 1997, 360-368). Reduced flow-rateionization sources make use of a tapered sprayer with an internaldiameter that is much smaller than those used in typical ESI sources.Reduced flow rate ion sources typically have a flow rate of 0.05 to 1.0μL/min and have a tapered sprayer with an internal diameter of 5-30 μm.Typical ESI and ionspray sources have flow rates of 1-1000 μL/min andsprayer tip diameters of 50-200 μm. For a given analyte concentration,the signal with a reduced flow-rate ion source is typically as great asor greater than that of conventional electrospray sources even thoughmuch lower flow rates are required. This is a result of the substantialincrease in the sampling efficiency of the analyte ions generated by thesource. Reduced flow-rate ion sources may also incorporate a nebulizergas flow. These types of ion sources are referred to as reducedflow-rate ionspray sources in the text that follows.

Another approach that can be used to increase the ion samplingefficiency of ESI for mass spectrometry involves modifying the massspectrometer to which the ESI source is attached. In particular, thediameter of the entrance aperture of the mass spectrometer may beincreased in order to draw more ions into the vacuum system. Providedthat the ion to gas ratio remains constant, an increase in the ionsignal is expected to be proportional to the increase in the gas flow.However, a larger vacuum pump will be required to maintain the samepressure within the mass spectrometer. Unfortunately, increasing thevacuum pump speed results in a mass spectrometer with a substantiallyhigher cost.

Prior art methods have looked at applying potentials in a vacuum regionor regions or a transition region or regions which are at reducedpressures to reduce the spread of the ions, i.e. to focus the ion beam.However, this is difficult because the ion spread is controlled by bothequipotentials and gas velocity within the reduced pressure region orregions. Also, if an inappropriate potential were applied to the lenselements, undesirable ion fragmentation may result. Conversely, in anatmospheric pressure region, it is the equipotentials which dominate theion trajectories and the distance that the ions travel betweencollisions is so short that the ions do not accumulate enough energy toeffect ion fragmentation or to achieve significant velocity.

Ion lenses have been used in vacuum regions to focus ion beams and alterion trajectories. Other prior art methods are directed towards improvingion trajectories immediately prior to entry into a downstream massspectrometer. Franzen et al. (U.S. Pat. No. 5,747,799) described a ringelectrode positioned on the inside wall of a heated capillary inlet,which was at or near atmospheric pressure, for a mass spectrometer thatwas downstream of an ESI source. The ring was intended to help draw ionsinto the inlet capillary of the mass spectrometer. The ring improved theshape of the equipotentials such that the electric field lines werepointed directly into the inlet capillary of the mass spectrometer.However, no evidence was given as to whether an appreciable increase inthe ion signal was observed.

Gulcicek et al. (U.S. Pat. No. 5,432,343) disclosed an interface for anESI source, at atmospheric pressure, connected to a mass spectrometerthat contained a transition region with multiple vacuum stages. Thetransition region included at least one electrostatic lens that had tobe properly positioned to aid in focusing the ions along a centerline.The electrostatic lens was intended to increase the ion transmissionefficiency through the second and third differentially pumped stages ofvacuum. In the ESI source housing, Gulcicek showed an end plate lenselement and a cylindrical lens which was placed near the perimeter ofthe housing of the ESI source. The lenses in the ESI source housing wereintended to help enrich the concentration of charged droplets near thecenterline, in the ESI source, where the desorbed analyte ions could bemore efficiently swept into a capillary entrance which led to thetransition region. However, these lenses were located at a substantialdistance from both the sprayer and the inlet aperture of the capillarythat led to the transition region so it is questionable as to how muchof a focusing effect the lenses in the source housing provided near thesprayer tip. While details of electric fields are given for other partsof the apparatus, no details are given of the electric field in thisatmospheric ionization chamber. Furthermore, no results were shown toindicate that an increase in ion signal is achievable with this method.

Feng et al. (Feng, X.; Agnes, G. R. J. Am. Soc. Mass. Spectrom. 2000,11, 393-399) evaluated several atmospheric pressure electrode designs toguide ions into the sampling orifice of a downstream mass spectrometer.The wire lenses were located downfield from a droplet levitation ionsource. The flow rate of the ion source was 5 μL/min. Feng et al. foundthat the wire lenses led to increased ion currents detected within amass spectrometer. However, the lenses used both AC and DC voltageswhich requires a more expensive power supply. Furthermore, the Fengdevice cannot be used with a curtain gas, therefore the practical use islimited. In addition, the Feng lens has been demonstrated to work onlywith single isolated droplets and not with a continuous ion source likean ESI source. Finally, the Feng lens is located in the desolvationregion substantially downfield from the source of ions.

Whitehouse et al. (U.S. Pat. No. 6,060,705) added windows along anatmospheric pressure ionization chamber to allow for direct viewing ofthe electrospray and the atmospheric pressure ion source duringoperation. Whitehouse also disclosed a cylindrical electrode extendingalong the side walls of the atmospheric pressure ionization chamber anda nebulizer gas flow which was applied to the electrospray needle tip.There were also three electrostatic lenses in a transition regionbetween the ion source and a downstream mass spectrometer. The potentialof the cylindrical electrode within the source housing was set so thatthe charged ions which left the electrospray needle tip were directedand focused by an electric field towards an orifice or capillaryentrance of the downstream mass spectrometer. Whitehouse noted thatthere was an increase in the ion signal when the potential applied tothe cylindrical electrode, within the source housing, was increased, aswell as when a potential was applied to the cylindrical lens and anebulizer gas was used to aid in breaking-up the charged droplets.Whitehouse also demonstrated that the potentials and the needle positioncould be adjusted to optimize the electrospray performance. However,once again, the cylindrical electrode within the ESI source housing wasfar away from the ESI sprayer. Furthermore, the configuration of thecylindrical electrode was fixed, and the position or orientation of theelectrode could not be adjusted.

Bertsch et al. (U.S. Pat. No. 5,838,003) disclosed an electrosprayionization chamber which operated substantially at or near atmosphericpressure and incorporated an asymmetric electrode. The asymmetricelectrode was either one half of a full cylinder, a flat semicircularplate, a wire or a flat circular disk. The sprayer was oriented at a 90degree angle to the axis of the ion entrance of the mass spectrometer.Bertsch also disclosed that the electrode may have extended past the tipof the sprayer. However, Bertsch demonstrated that the asymmetricelectrode was required to initiate and sustain the electrospray. Itappears that the asymmetric electrode is maintained at the samepotential as a counter electrode, i.e. similar to other prior proposalsthere is no clear teaching of a separate lens maintained at a potentialdifferent from that of two electrodes establishing the basic electricfield. Bertsch also taught that their device was applicable for flowrates of 1 μL/min up to 2 ml/min and thus was not applicable for reducedflow-rate ESI sources. Bertsch also stated that a nebulizer gas may beintroduced to assist in the formation of an aerosol.

In other work, Tang et al. (Tang, K.; Lin, Y.; Matson, D.; Taeman, K.;Smith, R. D. Anal. Chem. 2001, 73, 1658-1663) disclosed multiplemicroelectrospray emitters which successfully generated stablemultielectrosprays in a liquid flow rate range (1 to 8 μL/min totalflow) compatible with mass spectrometry. Higher total electrospray ioncurrents were observed as the number of electrosprays increased at agiven total liquid flow rate. Tang also disclosed that stableelectrosprays could be generated at higher liquid flow rates compared toconventional single ESI sources in which the electrospray was generatedfrom a fused-silica capillary. A nebulization gas may also be used withthe multiple microelectrospray emitters.

In light of the prior art, a need still remains for an inexpensiveapparatus that can be used to focus ions, as they are generated at thecapillary tip, to increase the ion flux into a downstream device such asa mass spectrometer. It is especially important to note that very fewstudies to date have focused on methods of improving ion trajectories asthe ions are generated in the sprayer plume of an ion source.

SUMMARY OF THE INVENTION

The present invention focuses on improving ion transmission into adownstream device, such as a mass spectrometer, by focusing on the pointat which the ions and charged droplets are initially generated. This isaccomplished by situating at least one “ion lens” in close proximity tothe sprayer tip of an ion source that is substantially at atmosphericpressure. In this document, “ion lens” or “ion focusing element” meansan electrode that can be used to change the equipotentials in theatmospheric pressure region in order to cause more ions from the sourceto reach a downstream device such as a mass spectrometer. Moreparticularly, the invention is concerned with an “ion lens” mountedadjacent a sprayer tip or a sprayer outlet, to change the equipotentialsas defined. Various shapes of ion lenses may be incorporated into theESI source to focus a larger number of ions into the orifice of thedownstream mass spectrometer. By adding a single ion lens and applying ahigh voltage to the ion lens, an increase in the total count rate of allions in the mass spectrum has been observed when a reduced flow-rate ESIsource and an ionspray source operating at high flow-rates were used. Inaddition, the ion signal stability was improved for both ion sources.Furthermore, the fragmentation and charge state patterns of the ionsproduced can be advantageously optimized by varying the geometry of theion lens (or ion lenses) and the magnitude of the potentials applied tothe ion lens (or ion lenses).

In a first aspect, the present invention provides an ion sourceapparatus for generating ions from an analyte sample, wherein theapparatus comprises an ion source, at least one counter electrode and anion focusing element. The ion source is mounted opposite the at leastone counter electrode and the ion focusing element is mounted relativeto the ion source. In use, a potential difference is applied between theion source and the at least one counter electrode to generate a spray ofionized droplets and to cause ions to move towards the at least onecounter electrode. In addition, a potential is applied to the ionfocusing element to change the equipotentials adjacent the ion source tofocus and direct ions in a desired direction of ion propagation. The ionfocusing element is located adjacent to the ion source such that theions are directed along an axis extending from the ion source. Thepotential applied to the ion focusing element is adapted to ensure thatthe equipotentials adjacent to the ion source are substantiallyperpendicular to the desired axis of ion propagation, both on the axisand for a substantial area around the axis.

In a second aspect, the present invention provides a method forgenerating ions from an analyte sample. The method comprises the stepsof:

1) supplying the analyte sample to an ion source;

2) providing at least one counter electrode spaced from the ion source;

3) providing a potential difference between the ion source and the atleast one counter electrode to generate a spray of ions or ionizeddroplets; and,

4) providing an ion focusing element and applying a potential to the ionfocusing element to change the equipotentials adjacent the ion source tofocus and direct ions in a desired axis of ion propagation.

The method further comprises providing the ion focusing element adjacentto the ion source such that the ions are directed along an axisextending from the ion source. The method further comprises adjustingthe potential applied to the ion focusing element to ensure that theequipotentials adjacent to the ion source are substantiallyperpendicular to the desired axis of ion propagation, both on the axisand for a substantial area around the axis.

It should be noted that in the present invention, an ion source is meantto comprise an ion sprayer. Furthermore, mass spectrometers typicallyhave an orifice plate with an orifice such that the ion source apparatusmay be bolted onto the orifice plate. Accordingly, a region is createdbetween the curtain plate of the ion source apparatus and the orificeplate in which curtain gas may be placed.

Further objects and advantages of the invention will appear from thefollowing description, taken together with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the present invention and to show moreclearly how it may be carried into effect, reference will now be made,by way of example, to the accompanying drawings which show preferredembodiments of the present invention and in which:

FIG. 1 is a simulation result showing equipotential lines andqualitative ion trajectories for a prior art conventional electrosprayion source operating at high liquid flow-rates;

FIG. 2 is a simulation result showing equipotential lines andqualitative ion trajectories for one preferred orientation of a priorart reduced flow-rate ESI source;

FIG. 3 is a simulation result showing equipotential lines andqualitative ion trajectories for a second preferred orientation of aprior art reduced flow-rate ESI source;

FIG. 4 a is a top view of a mounting device with an ion lens placed nearthe tip of a reduced flow rate ESI source in accordance with the presentinvention;

FIG. 4 b is a front view of the ion lens of FIG. 4 a placed on its sideand an attachment device for biasing the ion lens at a desiredpotential;

FIG. 4 c is a top view of the device of FIG. 4 a including a capillary;

FIG. 4 d is a front view of the ion lens of FIG. 4 c surrounding thecapillary tip from FIG. 4 c;

FIG. 5 a is a schematic of one embodiment of the ion lens;

FIG. 5 b is a schematic of an alternate embodiment of the ions lens inwhich the orifice of the ions lens is adjustable;

FIG. 5 c is a front view of the slotted window piece shown in FIG. 5 b;

FIG. 5 d is a front view of the cover piece which attaches the slottedwindow piece to the ion lens;

FIG. 6 a is a front view of a preferred embodiment of the location of anelectrospray capillary with respect to the ion lens;

FIG. 6 b is a side view of the preferred embodiment of the location ofan electrospray capillary with respect to the ion lens;

FIG. 6 c is a front view of a second preferred embodiment of thelocation of an electrospray capillary with respect to the ion lens;

FIG. 6 d is a side view of a second preferred embodiment of the locationof an electrospray capillary with respect to the ion lens;

FIG. 7 is a schematic of an embodiment of the present invention in whichan ion lens Is placed near the tip of an ionspray source;

FIG. 8 a is the mass spectrum obtained for a sample of reserpine using aprior art conventional ionspray source;

FIG. 8 b is the mass spectrum obtained for a sample of reserpine using aconventional prior art reduced flow-rate ESI source;

FIG. 8 c is the mass spectrum obtained for a sample of reserpine using areduced flow-rate ESI source incorporating an ions lens in accordancewith the present invention;

FIG. 9 a is the mass spectrum obtained for a sample of β-cyclodextrinusing a prior art conventional reduced flow-rate ESI source;

FIG. 9 b is the mass spectrum obtained for a sample of β-cyclodextrinusing a reduced flow-rate ESI source with an ion lens at a firstlocation in accordance with the present invention;

FIG. 9 c is the mass spectrum obtained for a sample of β-cyclodextrinusing a reduced flow-rate ESI source with an ions lens at a secondlocation in accordance with the present invention;

FIG. 10 a is a mass spectrum for β-cyclodextrin using a prior artconventional reduced flow-rate ESI source, and optimizing the source togenerate doubly protonated ions;

FIG. 10 b is a mass spectrum for β-cyclodextrin using a reducedflow-rate ESI source with an ion lens at a first location in accordancewith the present invention, and optimizing the source to generate doublyprotonated ions;

FIG. 10 c is a mass spectrum for β-cyclodextrin using a reducedflow-rate ESI source with an ion lens at a second location in accordancewith the present invention, and optimizing the source to generate doublyprotonated ions;

FIG. 11 a is a mass spectrum for cytochrome c using a prior artconventional ionspray source, and optimizing the source for maximum ionsignal;

FIG. 11 b is a mass spectrum for cytochrome c using a prior artconventional reduced flow-rate ESI source, and optimizing the source formaximum ion signal;

FIG. 11 c is a mass spectrum for cytochrome c using a reduced flow-rateESI source with an ion lens in accordance with the present invention,and optimizing the source for maximum ion signal;

FIG. 12 a is a mass spectrum showing the degree of fragmentation for asample of β-cyclodextrin using a reduced flow-rate ESI source with anion lens and a potential of 3750 V applied to the ion lens in accordancewith the present invention;

FIG. 12 b is a mass spectrum of the ion signal when the tip of the ionsprayer was moved closer to the curtain plate and the potential appliedto the ion lens was 5100 V in accordance with the present invention;

FIG. 12 c is a mass spectrum of the ion signal when the tip of the ionsprayer was positioned approximately flush to the curtain plate and thepotential applied to the ion lens was 4500 V in accordance with thepresent invention;

FIG. 13 is a simulation result showing equipotential lines andqualitative ion trajectories for a reduced flow rate ESI source with anion lens in accordance with the present invention;

FIG. 14 is a simulation result showing equipotential lines andqualitative ion trajectories for an ionspray source, or an electrospraysource operating at high liquid flow-rates, with an ion lens inaccordance with the present invention;

FIG. 15 is a graph of a signal measured in multiple ion mode whilemonitoring an ion signal using a prior art ionspray source without anion lens;

FIG. 16 is a graph of two signals measured in multiple ion mode whilemonitoring an ion signal using an ionspray source with an ion lens inaccordance with the present invention;

FIG. 17 is a graph of a signal measured in multiple ion mode whilemonitoring an ion signal using an ionspray source with an ion lens inaccordance with the present invention;

FIG. 18 is a graph of ion signal attenuation versus the horizontalposition of the sprayer of a prior art ionspray source without an ionlens and an ionspray source with an ion lens in accordance with thepresent invention;

FIG. 19 is a graph of ion signal attenuation versus the verticalposition of the sprayer of a prior art ionspray source without an ionlens and an ionspray source with an ion lens in accordance with thepresent invention;

FIG. 20 is a graph of ion signal intensity versus time during avariation of the operating parameters of the ion source whichincorporates an ion lens in accordance with the present invention;

FIG. 21 a includes three plots of ion signal intensity versus time asthe potential applied to the ions lens of an ion source is increased inaccordance with the present invention;

FIG. 21 b is a plot of total ion signal intensity versus time as thepotential applied to the ion lens of an ion source is increased inaccordance with the present invention;

FIG. 21 c is the mass spectra for the ion signal of FIG. 21 b obtainedat 0.433 minutes;

FIG. 21 d is the mass spectra for the ion signal of FIG. 21 b obtainedat 2.07 minutes;

FIG. 22 a is the mass spectra for a protein digest using a prior artreduced flow-rate ion source without an ion lens;

FIG. 22 b is the mass spectra for a protein digest using a reducedflow-rate ion source with an ion lens in accordance with the presentinvention;

FIG. 23 a is a graph of the ion intensity versus time and thecorresponding mass spectrum for a sample of glufibrinopeptide obtainedusing a standard prior art reduced flow-rate ion source without an ionlens;

FIG. 23 b is a graph of the ion signal intensity versus time and thecorresponding mass spectrum for a sample of glufibrinopeptide obtainedusing a standard reduced flow-rate ion source with an ion lens inaccordance with the present invention;

FIG. 24 a includes graphs of the ion signal intensity versus time andthe corresponding mass spectrum for one peptide in a digest of a 500fmol sample of beta-casein obtained using a prior art reduced flow-rateion source without an ion lens;

FIG. 24 b includes graphs of the ion signal intensity versus time andthe corresponding mass spectrum for one peptide in a digest of a 500fmol sample of beta-casein obtained using a reduced flow-rate ion sourcewith an ion lens in accordance with the present invention;

FIG. 24 c includes graphs of the background noise intensity versus timeand the ion signal intensity versus time for one peptide in a digest ofa 500 fmol sample of beta-casein obtained using a prior art reducedflow-rate ion source without an ion lens;

FIG. 24 d includes graphs of the background noise intensity versus timeand the ion signal intensity versus time for one peptide in a digest ofa 500 fmol sample of beta-casein obtained using a reduced flow-rate ionsource with an ion lens in accordance with the present invention;

FIG. 25 a is the mass spectrum for a triply charged peptide from abeta-casein digest obtained using a prior art reduced flow-rate ionsource without an ion lens;

FIG. 25 b is the mass spectrum for a triply charged peptide from abeta-casein digest obtained using a reduced flow-rate ion source with anion lens in accordance with the present invention;

FIG. 26 a is the background noise for a triply charged peptide (thesignal in FIG. 25 a) from a beta-casein digest obtained using a priorart reduced flow-rate ion source without an ion lens;

FIG. 26 b is the background noise for a triply charged peptide (thesignal in FIG. 25 b) from a beta-casein digest obtained using a reducedflow-rate ion source with an ion lens in accordance with the presentinvention;

FIG. 27 a is the mass spectrum for a doubly charged peptide from abeta-casein digest obtained using a prior art reduced flow-rate ionsource without an ion lens;

FIG. 27 b is the mass spectrum for a doubly charged peptide from abeta-casein digest obtained using a reduced flow-rate ion source with anion lens in accordance with the present invention;

FIG. 28 a includes graphs of the total ion chromatogram, base peakchromatogram, fragment ion chromatogram for the most dominant peptide ineach scan of the mass spectrometer and fragment ion chromatogram fromthe second most dominant peptide in each scan of the mass spectrometerversus time for a digest of a 100 fmol sample of bovine serum albuminobtained using a nano-high performance liquid chromatography (HPLC)-MSwith an ion source with an ion lens in accordance with the presentinvention;

FIG. 28 b is the mass spectra for a peptide and the fragment ions fromthe peptide from a digest of a 100 fmol sample of bovine serum albuminobtained using a nano-HPLC-MS mass spectrometer with an ion source withan ion lens in accordance with the present invention;

FIG. 29 is a graph of total ion signal intensity versus time for adigest of a 50 fmol sample of bovine serum albumin obtained using anano-HPLC-MS with an ion lens in accordance with the present invention;

FIG. 30 is a simulation result showing equipotential lines for an ionsource having two concentric ion lenses in accordance with the presentinvention;

FIG. 31 is a simulation result showing equipotential lines for an ionsource having two concentric ion lenses in accordance with the presentinvention;

FIG. 32 is a simulation result showing equipotential lines for the ionsource of FIG. 31 with the ion lenses slightly misaligned along the axisof the capillary in accordance with the present invention;

FIG. 33 is a simulation result showing equipotential lines for the ionsource of FIG. 31 with the ion lenses substantially misaligned along theaxis of the capillary In accordance with the present invention;

FIG. 34 is a simulation result showing equipotential lines for the ionsource of FIG. 31 with the ion lenses placed longitudinally along thesprayer in accordance with the present invention;

FIG. 35 is a schematic of a multispray ion source with an ion lens inaccordance with the present invention;

FIG. 36 is a simulation result showing equipotential lines for a priorart multispray ion source without an ion lens; and,

FIG. 37 is a simulation result showing equipotential lines for amultispray ion source with an ion lens in accordance with the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION

In this description, like elements in different figures will berepresented by the same numerals. In addition, all voltages are DCvoltages. Furthermore, all simulation results shown in this descriptionwere obtained using the MacSIMION, version 2.0 simulation program.

Simulation results for prior art ion source configurations will bedescribed first. Referring to FIG. 1, a conventional ionspray or highflow-rate ESI ion source 10 is shown comprising a sprayer 12, a curtainplate 14, an aperture 15 in the curtain plate 14, an orifice 16 in anorifice plate 18, a housing 20 and a sprayer mount 22. The curtain plate14, the orifice plate 18, and the housing 20 serve as counter electrodesfor the ESI ion source 10. The region between the curtain plate 14 andthe orifice plate 18 is at atmospheric pressure and is flushed with agas such as nitrogen. The rest of the interior of the housing 20 is alsoat atmospheric pressure. The orifice plate 18 separates the atmosphericpressure region in the housing 20 from any elements downstream from thehousing 20 such as the first stage of the vacuum system of a massspectrometer.

A simulation was conducted on this configuration in which the appliedpotentials were 5000 V on the sprayer 12, 1000 V on the curtain plate14, 190 V on the orifice plate 18 and 0 V for the housing 20 (it iscommon practice to maintain the housing at ground). The ESI sprayermount 22 was at the same potential as the sprayer 12. FIG. 1 shows thatthe equipotential lines, resulting from this arrangement of potentials,can be used to determine the direction of ion travel within the housing20. Ions experience a force in the direction of an electric field. Thedirection of the electric field within the housing 20 is perpendicularto a tangential line drawn at any point on the equipotential lines. Inan atmospheric environment, ions travel short distances betweencollisions and never gain substantial velocity. Hence, ion paths, in theabsence of a gas flow, can be determined by assuming that they arealways perpendicular to the equipotential lines. Accordingly, thecurvature of the equipotential lines at the tip of the sprayer 12 can beused to determine a series of ion trajectories such as 24 a, 24 b and 24c. As shown, these ion trajectories 24 a, 24 b and 24 c diverge over awide range and demonstrate the defocusing that the ions undergo afterthey leave the tip of the sprayer 12. With this arrangement, the spatialspread of the ions formed at the tip of the sprayer 12 increases as theions travel towards the curtain plate 14. This causes a large fractionof the generated ions to strike the curtain plate 14. Consequently, onlya very small fraction of the ions generated by the sprayer 12 passthrough the aperture 15 to reach orifice 16.

Referring to FIG. 2, a conventional reduced flow-rate ESI source 30 isshown with the tip of the sprayer 12 located much closer to the curtainplate 14 than the conventional ion source that was shown in FIG. 1. Thesprayer 12 is also centered in front of the inlet aperture 15. Asimulation was conducted on this configuration in which the appliedpotentials were 3000 V for the sprayer 12, 1000 V on the curtain plate14, 190 V on the orifice plate 18 and 0 V for the housing 20. Theequipotential lines, once more, result in a defocusing of the iontrajectories near the tip of the sprayer 12. The ion trajectories 34 aand 34 b illustrate that a widening plume 36 of ions is generated whichresults in a low efficiency of ion transfer through the orifice 16. Thisis because the spatial spread of ions formed at the tip of the sprayer12 becomes wider as the ions travel towards the orifice 16. Thiswidening of ion trajectories causes a large number of ions to strike thecurtain plate 14 or the orifice plate 18.

Referring to FIG. 3, an alternative arrangement for a conventionalreduced flow rate ESI source 40 is shown having the same componentsshown in FIG. 2. In this arrangement, the sprayer 12 is slightly offsetfrom the aperture 15 in the curtain plate 14. A simulation was performedusing the potentials from the simulation shown in FIG. 2. The simulationresults suggest a slight increase in ion signal sent through the orifice16 because there is a decreased spread of ions even though theequipotentials located near the tip of the sprayer 12 still appear to bedefocusing the ions. In this arrangement, the ions are directed at anangle that is sufficient to allow them to enter the orifice 16 moreefficiently.

The present invention will now be discussed. The present inventionprovides an ion focusing element, in close proximity to the ion sprayer,for focusing droplets or ions emitted from the capillary tip of an ionsource thereby improving the ion flux into a downstream device such as amass spectrometer or the like.

Referring to FIG. 4 a, an embodiment for a mounting device 50 for usewith reduced flow-rate ESI sources is shown. The mounting device 50comprises a sprayer mount 52 that is used to position an electrospraycapillary 66 (FIG. 4 b) and an ion lens 62. The sprayer mount 52 is madeof plexiglass. Alternatively, another non-conductive material may beused for the sprayer mount 52. The sprayer mount 52 has a mounting hole54, a groove 56, a conductive brass arm 58 and a set-screw 60 forsecuring an ion lens 62. The ion lens 62 may also be referred to as alens electrode or a ring electrode. The mounting hole 54 is positionedon the sprayer mount 52 so that the sprayer mount 52 may be installed oncommercial equipment, such as a mass spectrometer or the like, toreplace a commercial ionspray or electrospray arm. The groove 56 ismachined into the sprayer mount 52 to hold a stainless steel junction 64which is the point of application of a potential to the electrospraycapillary 66 to bias the tapered capillary tip 74 with respect to theion source housing (not shown), in which the sprayer mount 52 isinstalled. The ion source housing is typically held at 0 V. Thepotential is then applied to a capillary 66 through the conductive brassarm 58. The set-screw 60 is used to position the ion lens 62 at variouslocations. Alternatively, other types of bracketry or mountingarrangements could be used to keep the ion lens 62 in place.

Alternatively, the capillary 66 can be coupled with the tapered tip 74by any means known to those skilled in the art. This may include, but isnot limited to, a low dead volume conductive fastener in place of thestainless tube, a liquid junction (Zhang, B.; Foret, F.; Karger, B. L.Anal. Chem. 2000, 72, 1015-1022.), or a microdialysis junction (Severs,J. C.; Smith, R. D. Anal. Chem. 1997, 69, 2154-2158). In addition, theend of the capillary 66 may be pulled to a tapered tip. In this case,the electrospray potential may be applied using sheathless types ofinterfaces. These may include, but are not limited to applying aconductive coating to the sprayer tip (Wahl, J. H.; Gale, D. C.; Smith,R. D. J. Chromatogr. A. 1994, 659, 217-222 and Hofstadler, S. A.;Severs, J. C.; Swanek, F. D.; Ewing, A. G.; Smith, R. D. Rapid Commun.Mass Spectrom. 1996, 10, 919-923), or inserting an electrode into thesprayer (Cao, P.; Moini, M. J. Am. Soc. Mass Spectrom. 1997, 8, 561-564and Smith, A. D.; Moini, M. Anal. Chem. 2001, 73, 240-246). It will beapparent to those skilled in the art that there are many differentmethods for applying an electrospray potential to a reduced flow-rateion source, and the above methods are given as examples only, and are inno way meant to limit the scope or the spirit of this invention. Inaddition, any fastening means may be used to couple a capillary tip withany of the above junctions, including, but not limited to glue, a setscrew, a nut, an external clamp, or a compression fitting. In addition,the term microelectrospray can be used to describe reduced flow-rateelectrospray sources (Figeys, D.; Ning, Y.; Aebersold, R. Anal. Chem.1997, 69, 3153-3160).

Referring to FIG. 4 b, the ion lens 62 comprises two parts. The firstpart of the ion lens 62 is a ring 68 which is positioned around thecapillary 66. The second part of the ion lens 62 is an attachmentelement 70 which is adapted to bias the ion lens 62 at a desiredpotential.

Referring to FIG. 4 c, a reduced flow rate ESI source is shown whichcomprises the capillary 66 and the sprayer mount 52. The capillary 66and the tapered capillary tip 74 are connected inside the stainlesssteel junction 64 which is positioned on the groove 56. The tapered tip74 of the capillary 66 is preferably as uniform as possible in shape.The tapered tip 74 has an internal diameter of approximately 5-30 μm forreduced flow-rate applications. In a variety of embodiments thecapillary 66 may be connected to a syringe pump, a capillaryelectrophoresis instrument, a microfluidic device or any other type offluid delivery system compatible with the requirements of a reducedflow-rate ion source. A separate external power supply (not shown) isconnected to the ion lens 62 through a wire 72 for applying a potentialto the ion lens 62. This potential may be optimized depending on theliquid sample carried in the capillary 66, the solution flow-rate, thetype of solvent, the mass of the ions, the polarity of the ESI source,the electrospray potential, the curtain plate potential, the proximityof the sprayer to the curtain plate and the position of the ion lensrelative to the tip of the sprayer. In this embodiment, the end of thetapered tip 74 of the capillary 66 projects beyond the ion lens 62. Awire 24 is attached to a power supply (not shown) for application of theelectrospray potential.

Referring to FIG. 4 d, an end view of the ion lens 62 and the taperedtip 74 of the capillary 66 shows that the tapered tip 74 of thecapillary is preferably vertically centered in the ion lens 62 and nearthe left hand side of the ion lens 62 in one favorable embodiment. In analternative favorable embodiment, the tapered tip 74 is preferablyvertically centered in the ion lens 62 and horizontally centered in theion lens 62. Alternatively, the tapered tip 74 may be asymmetricallyplaced, both horizontally and vertically, within the ion lens 62.Furthermore, the plane defined by the ion lens is positionedsubstantially perpendicular to the axis of the capillary 66 and the tip74 of the capillary 66 abuts or intersects this plane. The position ofthe ion lens is also adjustable along the axis of the capillary 66. Theposition of the ion lens is preferably optimized to maximize the ionflux into a downstream device such as a mass spectrometer. Optimizationinvolves adjusting the position of the sprayer and setting thepotentials applied to the various components of the ion source.

Referring to FIGS. 5 a and 5 b, these Figures show two other embodiments62′ and 62″ of the ion lens 62. The physical dimensions, all in mm, areshown for illustrative purposes only. Accordingly, other dimensions andshapes may be used. In FIG. 5 a, the ion lens 62′ is non-adjustable. Theion lens 62′ preferably has a length of 19 mm, and a height of 8 mm andan aperture 76′ with slightly smaller dimensions. The aperture 76′preferably has a length of 10 mm and a height of 5 mm. The ion lens 62′also has a thickness of 1 mm and is made from stainless steel. Otheraperture dimensions ranging from 5 mm to 15 mm have been used to achievefavorable results as well. In general, the smallest dimensions for theion lens 62 are dictated by the onset of arcing to the sprayer and thelargest dimensions for the ion lens 62 are dictated by spatiallimitations and decrease in effectiveness. The ion lens 62 may beconstructed of other conductive materials as well, however, stainlesssteel is used because it is inert.

Referring to FIG. 5 b, ion lens 62″ is adjustable in that the size ofaperture 76″ can vary in size in the horizontal direction due to aslotted window piece 78. To increase the size of the aperture 76″, theslotted window piece 78 is moved to the right. Likewise, to decrease thesize of the aperture 76″, the slotted window piece 78 is moved to theleft. The size of the aperture 76″ of the ion lens 62″ is adjustable sothat the ion signal may be optimized. In this embodiment, the verticaldimension of the ion lens 62″ is non-adjustable, however, a verticaladjustment could easily be built into the ion lens 62″ in an alternateembodiment.

The slotted window piece 78 is shown in more detail in FIG. 5 c. In apreferred embodiment, the slotted window piece 78 has a groove 80 whichis used to permit horizontal movement of the slotted window piece 78.The slotted window piece 78 is slid into a horizontal groove (not shown)in the ion lens 62″. The horizontal groove allows the slotted windowpiece 78 to be moved in the horizontal direction, effectively changingthe size of the ion lens aperture 76″. Alternatively, a series of ionlenses with different dimensions may be used. In an alternativeembodiment, the length of the aperture 76″ is adjustable from a lengthof 7 mm to a length of about 14 mm although a length of 9 mm may bepreferable. A cover piece 81 is placed over the slotted window piece 78and a screw, through aperture 82, holds the cover piece 81 and theslotted window piece 78 onto the ion lens 62″.

The ion lens 62 is annular and has a solid cross section. Alternatively,the “ring” of the ion lens 62 may be hollow. The ion lens 62 may furtherhave a continuous or discontinuous cross-section having the form of acircle, an oval, a square, a rectangle, a triangle or any other regularor irregular polygonal shape or other two-dimensional shape. Note thatthere may also be a gap in the “ring” portion of the ion lens 62 so thatthe ion lens 62 substantially surrounds the sprayer.

Referring to FIGS. 6 a and 6 b, a preferred embodiment of the positionof the tapered tip 74 of the capillary 66 is shown. Experimental resultswhich support this embodiment are discussed later on. In thisembodiment, the ion lens 62 is positioned horizontally asymmetric withrespect to the tapered tip 74 of the capillary 66. The tapered tip 74 ofthe capillary 66 is approximately 2 mm from the right hand side of theion lens 62 and approximately 7 mm from the left hand side of the ionlens 62. In the vertical direction, the tapered tip 74 of the capillary66 is centered within the ion lens 62.

Referring to FIGS. 6 c and 6 d, a second preferred embodiment of theposition of the tapered tip 74 within the ion lens 62 is shown.Experimental results which support this embodiment are also discussedlater on. In this embodiment, the ion lens 62 is horizontally andvertically centered with respect to the tapered tip 74 of the capillary66. The positioning of the tapered tip 74 within the ion lens 62 may beoptimized to increase the ion flux, and the position of the sprayermount 52 may be adjusted with respect to the aperture 15 in the curtainplate 14, i.e. the distance from the sprayer mount 52 to the curtainplate 14, whether the sprayer mount 52 is aligned with the aperture 15in the curtain plate 14 or whether the sprayer mount 52 is offset fromthe aperture 15 in the curtain plate 14 and the like. This optimizationprocess would also include varying the potentials on the variouscomponents of the ion source.

It has also been found that the position of the ion lens 62 along theaxis of the capillary 66 with respect to the end of the tapered 74affects the generated ion signal. The ion lens 62 is preferablypositioned approximately 0.1 to 5 mm behind the end of the tapered tip74. More preferably, the ion lens 62 may be positioned approximately 1to 3 mm behind the end of the tapered tip 74. Most preferably, the ionlens 62 is placed approximately 2 mm behind the end of the tapered tip74 as shown in FIG. 6 b. The effectiveness of the ion lens 62 may varyas the ion lens 62 is moved farther forward or back from 2 mm behind theend of the tapered tip 74. Furthermore, it may be preferable to applylarge potentials to the ion lens 62 to increase the focusing of thegenerated ions. However, due to the loss of spraying efficiency, as theion lens potential increases, the effective electric field at the tip 74of the sprayer 12 seems to decrease. Eventually, the electric field isnot large enough to produce a stable electrospray.

Reference is now made to an embodiment of an ionspray, or high flow-rateelectrospray ionization source 90 with an ion lens 62 shown in FIG. 7.The ionspray source 90 preferably comprises a sprayer mount 52, amounting hole 54, a set screw 60, a capillary 66, an ion lens 62, anadjustable support 92, a turnable mount 94, a Teflon arm 96, a sprayer98, a stainless steel tee 100 and tubing 102. The sprayer mount 52 issimilar to that used in some commercial ionspray sources with a mountinghole 54 which is adapted to attach the sprayer mount 52 to a commercialtype of stud mount (not shown). The adjustable support 92 is attached tothe sprayer mount 52 via the setscrew 60. The adjustable support 92 isattached to the sprayer mount 52 to optimize the position of the ionlens 62 relative to the sprayer 98 and more particularly to the tip 99of the sprayer 98. The turnable mount 94 and the Teflon arm 96 are usedto hold the ion lens 62 in place. The turnable mount 94 may be rotatedthrough 360 degrees which allows for the precise angle of the ion lens62 relative to the sprayer 98 to be adjusted. The length of the Teflonarm 96 may range from 1 to 20 cm depending on the required distance forpositioning the ion lens 62 relative to the tapered tip 99.

In use, an analyte solution travels via the capillary 66 to a stainlesssteel tee 100. A nebulizer gas, which is carried to the stainless steeltee 100 via the tubing 102, flows coaxially through a stainless steeltube which surrounds capillary 66. The nebulizer gas consists ofcompressed air, but may be replaced with nitrogen, oxygen, sulphurhexafluoride, or other gases. In particular, nebulizer gases such asoxygen and sulphur hexafluoride may be useful as electron scavenginggases when operating in negative ion mode. The analyte solution in thecapillary and the coaxial nebulizer gas travel through the sprayer 98 tothe sprayer tip 99. The nebulizer gas assists in breaking up chargeddroplets at the sprayer tip 99. The nebulizer gas also allows for muchhigher analyte solution flow-rates to be used and may help to evaporatethe solvent in the analyte sample. A potential is applied to the ionlens 62 to focus the charged droplets (that are forming) into a narrowion beam which is directed to an aperture associated with thecounter-electrode for the ionspray ionization source 90. In a preferableembodiment, the ion lens 62 has an aperture with a height of 6 mm and alength which is adjustable from 6 mm to 12 mm. Other preferredembodiments of the ion lens 62 include oblong shapes with dimensions of12.4 mm ×8.90 mm, 14.10 mm ×10.2 mm, 14.92 mm ×11.10 mm, 17.60 mm ×13.00mm and 19.3 mm ×15.00 mm. Other dimensions may also be used. It isimportant to note that the ion lens 62 would be effective for use with aturbo-ionspray source as well. In turbo-ionspray sources, an additionalflow of heated gas is directed at the electrospray plume to assist inevaporating the droplets and in desolvating ions. This turbo-ionspray isdescribed in U.S. Pat. No. 5,412,208 which is hereby incorporated byreference.

Reference is now made to FIGS. 8 a- 8 c which depict the ion signalincrease achieved when using an ion source with an ion lens on a massspectrometer with a sample of reserpine. FIG. 8 a shows the massspectrum obtained with a commercial ionspray source without an ion lens,FIG. 8 b shows the mass spectrum obtained with a reduced flow-rate ESIsource without an ion lens and FIG. 8 c shows the mass spectrum obtainedwith a reduced flow-rate ESI source with an ion lens. The solution flowrate was 1 μL/min for the commercial ionspray source and 0.2 μL/min forthe reduced flow rate ESI sources. The reserpine sample was preparedwith a concentration of 10⁻⁵ M in a solution of 10% water and 90%acetonitrile with 1 mM ammonium acetate. The reserpine sample wasprepared in a mostly volatile non-aqueous matrix and therefore a verylarge potential, relative to the sprayer potential, could be maintainedon the ion lens which resulted in a strong ion signal. The voltageparameters for the experiment of FIG. 8 c were 4000 V, 2000 V, and 5700V for the reduced flow rate sprayer, curtain plate, and ion lensrespectively. In FIG. 8 a, the voltage parameters were 5000 V and 1000 Vfor the sprayer and the curtain plate, respectively. In FIG. 8 b, thevoltage parameters were 3000 V and 1000 V for the sprayer and curtainplate, respectively.

The ion signals 104 and 106 obtained in FIGS. 8 a and 8 b respectivelywere quite similar although a slightly higher ion signal 106 wasobtained with the reduced flow-rate ESI source. However, FIG. 8 c showsthat a significant enhancement for the ion signal 108 is obtained whenan ion lens is used. The ion signal 108 is approximately 2 to 2.5 timesstronger than the ion signals 104 and 106 with the ion lens in place.There is also a substantial increase in the solvated ion peaks 112 inthe mass spectra as well.

Reference is now made to FIG. 9 which depicts the ion signal increaseachieved when using an ion source with an ion lens on a massspectrometer with a solution of 10⁻³ M of β-cyclodextrin. FIG. 9 a showsthe mass spectrum obtained with a reduced flow-rate ESI source withoutan ion lens, FIG. 9 b shows the mass spectrum obtained with a reducedflow-rate ESI source with an ion lens in a first position and FIG. 9 cshows the mass spectrum obtained with a reduced flow-rate ESI sourcewith an ion lens in a second position. In FIG. 9 b, the sprayer wasapproximately 2 mm from the curtain plate and in FIG. 9 c the sprayerwas approximately 1 mm from the curtain plate. All mass spectra wereobtained from the summation of 10 scans.

These Figures demonstrate an Increase in the total number of ions fromthe β-cyclodextrin sample when an ion lens is used. In FIGS. 9 a- 9 c,β-cyclodextrin with an ammonium adduct is the dominant peak (i.e. peaks114, 116, 118 in FIGS. 9 a- 9 c) at a mass-to-charge (m/z) ratio of1153. The next dominant peak is protonated P-cyclodextrin at a m/z ratioof 1136 (i.e. peaks 120, 122 and 124 in FIGS. 9 a- 9 c). The peaks atm/z ratios of 326, 488, 650, 812, and 974 are fragment peaks. Anincrease in the parent ion signal, peaks 118 and 116 versus 114, of 2.5to 3 times is seen in FIGS. 9 b and 9 c where an ion lens was used.Furthermore, in FIGS. 9 b and 9 c there is also an increase of everyfragment peak by a factor of 3.5 to 5.5. These fragments correspond tolosses of successive glucose molecules from β-cyclodextrin due tocollisions with gas molecules within the first differentially pumpedvacuum stage of the mass spectrometer. The results shown in FIGS. 9 band 9 c were obtained with applied potentials of 3000 V on both thereduced flow rate sprayer and the ion lens, 190 V on the orifice plateand slightly more than 1000 V on the curtain plate. In FIG. 9 a, thepotentials were 3000 V, 1000 V and 190 V for the sprayer, curtain plateand orifice plate, respectively.

In the experiments in which an ion lens was added to a reduced flow-rateESI source at substantially atmospheric pressure, it was found that thestrength of the ion beam was optimized when the ion lens was locatedapproximately 0.1 to 5 mm and more preferably 1.5-3 mm behind the end ofthe tapered tip of the capillary. In some instances it was alsopreferable to place the ion lens around the tapered tip of the capillarywith an asymmetrical orientation in the horizontal direction as shown inFIG. 6 b. The horizontal distance from the tapered capillary to theright side of the oblong-shaped aperture of the ion lens wasapproximately 2 mm. The distance from the capillary to the left side ofthe oblong-shaped aperture of the ion lens was approximately 7-8 mm. Inthe vertical direction, the capillary was preferably centered in theaperture of the ion lens; i.e. the spacing between the capillary to thetop and the bottom of the aperture of the ion lens was approximately 2.5mm. For this embodiment, the reduced flow-rate ESI sprayer waspositioned close to the right hand edge of the aperture in the curtainplate. Similar results could be obtained by placing the tapered tipcloser to the left hand side of the ion lens, and positioning thesprayer close to the left hand side of the aperture in the curtainplate, or by turning the ion lens at a 90 degree angle and orienting thesprayer near the top or the bottom of the aperture in the curtain plate.In other instances, it was preferable to place the ion lens around thetapered tip of the capillary with a symmetrical orientation in both thehorizontal and vertical direction as shown in FIG. 6 d. In thisembodiment, the sprayer was centered in front of the aperture in thecurtain plate. The end of the capillary Up was either centered in frontof the aperture, or off to the side. To achieve optimal results, it waspreferable that the shape of the tapered tip of the capillary was asuniform as possible since the beneficial effects of the ion lensdecreased when a capillary with a damaged tip was used. Other testsshowed that an asymmetric placement of the tapered tip in the ion lens(in both dimensions) showed favorable results.

The test results of the ion lens with a reduced flow rate ESI source atsubstantially atmospheric pressure showed a significant increase in thetotal ion count. In fact, the use of an ion lens with a reducedflow-rate ESI source increased the total number of ions entering themass spectrometer by a factor of approximately three or four compared tothe reduced flow-rate ESI source alone. For instance, the total countrate for all ions in the mass spectrum of a β-cyclodextrin sample usinga commercial ionspray source without an ion lens was approximately 1.3million counts per second (cps) whereas the total ion count for thesample using the reduced flow-rate ESI source with the ion lens resultedin a total ion count of approximately 5.5 million cps. In theexperiments with the reduced flow-rate ESI source with the ion lens, thesprayer was located very close to the curtain plate whereas in theexperiments without the ion lens, the sprayer had to be positionedfarther away from the curtain plate to maintain a strong signal.

Reference is now made to FIGS. 10 a- 10 c which depict changes in thecharge state for a particular compound when using an ion source atsubstantially atmospheric-pressure with an ion lens on a massspectrometer for a sample of β-cyclodextrin. FIG. 10 a shows the massspectrum obtained with a reduced flow-rate ESI source without an ionlens and FIGS. 10 b and 10 c show the mass spectra obtained with thereduced flow-rate ESI source with an ion lens. The β-cyclodextrinsolution comprised 10⁻⁵ M β-cyclodextrin in approximately 10 mM ammoniumacetate at a pH of 7. The results in each of these Figures were achievedwith an applied potential of 140 V on the orifice plate.

Referring to FIG. 10 a, the applied voltages were 3000 V on the ESIsprayer, and 1000 V on the curtain plate. In this Figure, the singlycharged β-cyclodextrin 126 at a m/z ratio of 1153 is the predominant ionspecies observed in the mass spectrum. In FIGS. 10 b and 10 c, theapplied potentials were 3000 V for the sprayer, 1580 V for the curtainplate and 2850 V for the ion lens. In addition, the Up of the reducedflow-rate sprayer was positioned very close to the curtain plate. Thetip of the reduced flow-rate sprayer was also moved slightly closer tothe middle of the aperture of the curtain plate for FIG. 10 c as opposedto FIG. 10 b. It can be seen that with the addition of the ion lens, thedoubly charged peak 128 and 132 at a m/z of 586 can be increasedrelative to the other peaks in the mass spectrum. The ion signals arealso substantially increased, with a 3.3 times increase in totalβ-cyclodextrin ions detected even though the singly charged peak 130 and134 is only slightly changed from the peak 126 in FIG. 10 a. For FIG. 10a, it was not possible to generate a greater degree of doubly chargedβ-cyclodextrin ions. It is Important to note that this increase in theion signal for the doubly charged β-cyclodextrin is achieved while onlyslightly reducing the ion signal for the singly charged molecule.

The ability of the ion lens to vary the charge state of a particular ionis also seen in FIGS. 11 a- 11 c which illustrate the mass spectraobtained for an ion source with a mass spectrometer analyzing a solutionof the protein cytochrome c. FIG. 11 a is a mass spectrum obtained withan ionspray ion source without an ion lens, FIG. 11 b is a mass spectrumobtained with a reduced flow-rate electrospray ion source and FIG. 11 cis a mass spectrum obtained with the reduced flow-rate electrospray ionsource with an ion lens. The solution comprises cytochrome c at aconcentration of 100 μmol/L in water with approximately 1% acetic acid.The peaks in the mass spectra of FIGS. 11 a- 11 c correspond to thevarious charge states of the protein cytochrome c. The peak 136, at am/z ratio of 1547, corresponds to a charge state of +8; the peak 138, ata m/z ratio of 1375, corresponds to a charge state of +9 and the peak140, at a m/z ratio of 1238, corresponds to a charge state of +10. Inall cases, the ion sources were adjusted to yield the largest ionsignal. The addition of the ion lens allows for selective enhancement ofthe ion signal for the protein with a particular charge state. Theapplied potentials for the ionspray source without the ion lens (FIG. 11a) were 4796 V for the sprayer and 1000 V for the curtain plate.Furthermore, a nebulizer gas was used with a pressure of 30 psi. For thereduced flow-rate ion source without the ion lens (FIG. 11 b), theapplied potentials were 3374 V for the sprayer and 1560 V for thecurtain plate. For the reduced flow-rate ion source with the ion lens(FIG. 11 c), the applied voltages were 4000 V on the sprayer, 2000 V onthe curtain plate and 4200 V on the ion lens. All other parameters ofthe mass spectrometer were constant for the mass spectra of FIGS. 11 a-11 c.

The ability to vary the charge states can be effected by varying thepotential applied to the ion lens and the position of the sprayerrelative to the aperture in the curtain plate. In fact, for sugars andproteins, higher potentials applied to the ion lens may be effective forgenerating or focusing higher charge state ions into a massspectrometer. Experiments conducted with bradykinin demonstrate theability of the ion lens to substantially increase the ion signal for thehigher charge states of peptides (+2 and +3) while at the same timedecreasing or maintaining the signal for the singly charged backgroundsolvent peaks. This can lead to substantial increases (i.e. a factor of3 to 6) for the signal to noise ratio of the multiply charged peptidepeaks.

The use of an ion lens may also result in a variation of the degree offragmentation of the parent ions in an analyte sample. Referring now toFIGS. 12 a- 12 c, the mass spectra obtained with a reduced flow-rate ESIsource with an ion lens on a mass spectrometer are shown. The sample wasβ-cyclodextrin, as described previously for FIGS. 9 a- 9 c. In each ofthese Figures, the results were obtained with applied potentials of 190V on the orifice plate, 1000 V on the curtain plate, 3100 V on thesprayer and 110 V on a skimmer within the first vacuum stage of adownstream mass spectrometer. The applied potential to the ion lens was3750 V, 5100 V, and 4500 V for FIGS. 12 a- 12 c respectively. Theincrease in the applied potential on the ion lens allows the sprayer tobe positioned slightly closer to the aperture of the curtain plate. Foreach Figure, the sprayer was positioned in front of the aperture and thecurtain gas flow rate was constant. For FIG. 12 c, the tip of thesprayer was positioned approximately even with the curtain plate. ForFIGS. 12 a and 12 b, the ion lens was positioned approximately 2 mmbehind the tip of the reduced flow rate sprayer. For FIG. 12 c, the ionlens was moved even farther behind (approximately 4 mm behind) the tipof the reduced flow-rate sprayer to allow the tip of the reducedflow-rate sprayer to be placed approximately even with the curtain platewithout arcing between the ion lens and the curtain plate. The peaks atm/z ratios of 326, 650, 488, 812 and 974 correspond to fragment ionsgenerated by collision-induced dissociation in the first differentiallypumped vacuum region of a downstream triple quadrupole massspectrometer. The fragment ion peaks decrease in magnitude as the ionspray Is generated closer to the inlet aperture of the massspectrometer. This data demonstrates that the degree of ionfragmentation can be varied by adjusting the position of the sprayer tiprelative to the curtain plate and setting an appropriate lens potential.

It is not clear at this point whether the variation in the mass spectrumis due to a change in the mechanism of the electrospray itself or due tothe fact that the charged droplets are forming closer to the aperture ofthe curtain plate which may cause a higher degree of solvation on thegas phase ions in FIGS. 12 b and 12 c. A higher degree of ion solvationnecessitates an increased internal input energy between the orificeplate, and the skimmer, in a downstream mass spectrometer, to achievedesolvation. Thus, less energy would be available for ion fragmentationfor a fixed potential difference between the orifice plate and theskimmer in the mass spectrometer. An increase in solvation is consistentwith the increased signals experimentally observed for the solvated ionsin other mass spectra as well such as in FIG. 8 c. The spacing on someof the peaks above the reserpine peak (a m/z ratio of 609) was 18 m/zratio units which suggests that some of the increased ion signal was dueto higher order solvation.

The increase in ion signal due to the use of an ion lens may be due to achange in the equipotentials near the tip of the sprayer. Referring nowto FIG. 13, the results of a simulation of a reduced flow-rate ESIsource with an ion lens 62 is shown. For the simulation, the appliedpotentials were 5100 V for the ion lens 62, 3500 V for the sprayer 12,2000 V for the curtain plate 14, 190 V for the orifice plate 18 and 0 Vfor the housing 20. The simulation results show that the shape of theequipotentials generated near the tip of the sprayer 12 have improvedwhen an ion lens 62 is placed near the tip of the sprayer 12. Theequipotentials at the tip of the sprayer 12 are flatter compared to theequipotential lines near the tip of the sprayer 12 in FIG. 2.Accordingly, the resulting electric field lines near the tip of thesprayer 12 result in ion trajectories 160 which point directly to theaperture 15 in the curtain plate 14. The configuration of FIG. 13reduces the spread of ion trajectories and directs the ion trajectoriesin the general direction of the desired axis of ion propagation. Thisresults in a reduction of the defocusing effect observed in FIG. 2.Thus, more ions are guided towards the orifice 16 of a downstream devicesuch as a mass spectrometer (not shown).

Reference is next made to FIG. 14 which shows the result of a simulationdone on an ion lens positioned near the vicinity of the sprayer of anion source which was at substantially atmospheric pressure, similar tothe ion source shown in FIG. 1. The applied potentials in thissimulation were 5000 V for the sprayer 12, 5000 V for the ion lens 62,1000 V for the curtain plate 14, 190 V for the orifice plate 18 and 0 Vfor the housing 20. The potentials applied to the sprayer 12 and the ionlens 62 are equal in this example but this does not necessarily have tobe the case. FIG. 14 shows that the equipotential lines near the tip ofthe sprayer 12 are relatively flat which causes the trajectories of thegenerated ions to be more confined along an axis of propagation 162. Inthis simulation, the tip of the sprayer 12 is not aligned with theaperture 15 in the curtain plate 14, however, the ion signal transmittedto the orifice 16 is increased. In this embodiment, the sprayer 12 isoriented on approximately a 45 degree angle relative to the curtainplate, but it will be apparent to those skilled in the art that otherorientations will be equally effective.

Experiments were also conducted to determine the effect of the ion lenson the stability of the ion signal. The experiments showed that the useof an ion lens resulted in a stabilization of the ion signal monitoredin a mass spectrometer over time. The stability of the ion signal wasmeasured using the relative standard deviation of the ion signalobtained for repeated measurements taken in 10 ms intervals. Themeasurements showed that with conventional ionspray sources, therelative standard deviation is approximately 2 times higher than thatachieved with an ion lens. It was also found that there was a reduceddependence of the ion signal upon the location of the sprayer relativeto the aperture in the curtain plate which made optimizing the locationof the sprayer within the source housing much easier. These results willnow be discussed.

In the experiments, an ionspray source was constructed to resemble theionspray source shown in FIG. 7. The outer diameter at the tip 99 of thesprayer 98 was approximately 450 μm. The sprayer housed a fused silicacapillary with an outer diameter of approximately 150 μm and an innerdiameter of approximately 50 μm. A solution flow rate of between 1 and 4μL/min was used. The sample used in the experiment was a 1 mM solutionof β-cyclodextrin in water with 10 mM ammonium acetate at a pH of 7. Thesprayer was located approximately 7.5 mm from the curtain plate. Thepotentials applied to the sprayer and the curtain plate wereapproximately 6000 V and 1800 V respectively. The experiments showedthat it was preferable to apply a potential of 2500 to 5000 V to the ionlens and that it was not possible to maintain an ion signal whenpotentials greater than 5000 V were applied to the ion lens. Theionspray source was used with a conventional triple quadrupole massspectrometer to analyze the ion signal which was produced by theionspray source.

Experimental results for a sample of β-cyclodextrin in ammonium acetateshowed that the predominant peak in the mass spectrum was cyclodextrinwith an ammonium adduct at a m/z ratio of 1153. The experimental resultsalso showed that the ion lens improved the short-term stability of theion signal as determined by the Relative Standard Deviation (RSD) ofrepeated measurements. In fact, the RSD was decreased by a factor ofapproximately 2 for an ionspray source with an ion lens compared to aconventional ionspray source without an ion lens. The ion lens alsoallowed for a more precise calculation of the ratio of peaks in the massspectrum. In addition, the magnitude of the ion signal increased by afactor of approximately 1.5.

In particular, Table 1 shows a comparison of the signal stabilitybetween an ionspray source without an ion lens and an ionspray sourcewith an ion lens over a measurement period of approximately 15 minutes.The m/z ratio range from 800 to 1200 was scanned with a dwell time of 10ms. Twenty repeat runs were averaged to obtain the standard deviation ofthe measured ion signal. Each of the twenty runs was the result of 10scans. For each of these runs, the sprayer and ion path parameters wereoptimized to obtain as stable an ion signal as possible. In this case,the source with the ion lens is tuned to produce a similar signalintensity to that of the ionspray source without the ion lens. Anaverage RSD of slightly less than 3% was obtained for the ionspraysource without the ion lens. The addition of the ion lens reduced theRSD by a factor of approximately 2.0. However, there is still someinstability from the source. The last row of Table 1 shows the RSD thatwould be obtained if the source was completely stable (i.e. if the RSDwas determined purely by ion counting statistics).

TABLE 1 Comparison of the Signal Stability Measurement Ionspray Ionspraywith an Parameter (Best Run) Ion Lens Number of 20 20 MeasurementsAverage Signal (cps) 1.857 × 10⁶ 1.663 × 10⁶ RSD (%) 2.84 1.41 RSD ofCount 0.55 0.58 Statistics (%)

Reference is next made to Table 2, which shows that the ion lensimproved the ability to obtain the ratio of two peaks in a massspectrum. In the experiment, the two peaks corresponded to protonatedcyclodextrin at a m/z ratio of 1136 and cyclodextrin with an ammoniumadduct at a m/z ratio of 1153. The peak at a m/z ratio of 1136 wasgenerated by collisions within the region between the orifice and theskimmer of the downstream triple quadrupole mass spectrometer. Sixrepeat measurements were made to determine the average ratio of theaforementioned peaks. Table 2 shows that typical RSD values for anionspray source without the ion lens were slightly greater than 3%.However, the addition of the ion lens near the tip of the ionspraysource reduced the RSD to approximately 1.4%. Thus, an ionspray sourcewith an ion lens may be used to improve precision in applications whichrequire the accurate reading of ratios of peaks in a mass spectrum suchas in determining isotope ratios. Again, there is still some instabilityfrom the source. The last row of Table 2 shows the RSD that would beobtained if the source was completely stable (i.e. if the RSD wasdetermined purely by ion counting statistics).

TABLE 2 Comparison of the ratio of two peaks in the mass spectrumIonspray Ionspray with an Ion Lens Orifice-Skimmer Potential 58 V 58 VDifference (V) Number of Measurements 6 6 Ratio Average 17.6 12.3 RatioRSD (%) 2.97 1.40 Count Stats RSD (%) 1.17 1.17

Referring now to Table 3, the RSD was calculated by performing anexperimental trial that involved-taking 1498 readings (using a 10 msecdwell time) of the magnitude of the peak for cyclodextrin with anammonium adduct over a time period of 1 minute. The sample flow rate was4 μL/min. The data presented is the average of four trials. Table 3shows that the ion signal is increased by a factor of slightly greaterthan 1.5 and the RSD is reduced from approximately 4.1% to approximately2.6% for an ionspray source with an ion lens as compared to an ionspraysource without an ion lens. Again, there is still some instability fromthe source. The last row of Table 2 shows the RSD that would be obtainedif the source was completely stable (i.e. if the RSD was determinedpurely by ion counting statistics.)

TABLE 3 Comparison of the Signal Stability Ionspray Ionspray with an IonLens Replicates 1498 1498 Average Ion Signal (cps) 3.707 × 10⁵ 5.645 ×10⁵ Average RSD (%) 4.10 2.64 Count Stats RSD (%) 0.04 0.04

The ion stability achievable for an ionspray with an ion lens is alsoshown in FIGS. 15-17. The data was collected in the multiple ion modewhile monitoring an ion signal for cyclodextrin ions, at a m/z ratio of1153, and protonated cyclodextrin, at a m/z ratio of 1136. In FIGS.15-17, the vertical axis is the log (base 10) of the ion signalcalculated as ions per second and the horizontal axis is the measurementnumber. There are 3000 measurements of 10 ms each, so the horizontalaxis ranges from 0 to 30 s. FIG. 15 shows a graph of the signal versustime obtained in multiple ion mode while monitoring an ion signal forcyclodextrin at a m/z ratio of 1152 using an ionspray source without anion lens. The signal is very “choppy” which makes it difficult to obtainan accurate measurement. FIG. 16 shows the signal from an ionspraysource with an ion lens that is obtained in multiple ion mode whilemonitoring the ion signals at m/z ratios of 1152 and 1135. These signalsare more stable. FIG. 17 shows the signal from the ionspray source withan ion lens that is obtained after further optimization of the potentialof the ion lens and the position of the ion lens while monitoring theion signal at a m/z ratio 1152. This signal is also more stable.

Reference is next made to FIG. 18 which shows a graph of ion signalversus the position of the sprayer of an ionspray source, atsubstantially atmospheric pressure, relative to the right hand side ofthe aperture in the curtain plate. The data is shown for an ionspraysource without an ion lens (diamond shaped data points) and an ionspraysource with an ion lens (square shaped data points). FIG. 18 shows thatthe ion lens makes the ionspray source easier to operate since the ionsignal is not attenuated as much for the ion source with an ion lenscompared to the ion source without an ion lens when the position of thesprayer changes. In FIG. 18, the point along the x axis defined as 0 mmis the point where the sprayer is located at the very right hand edge ofthe aperture in the curtain plate. The distance from the aperture wasmeasured with a ruler attached to the top of the source housing.

FIG. 18 shows that the ion signal remains approximately constant (90% ofthe maximum ion signal, i.e. the ion signal at 0 mm) as the sprayer, ofthe ionspray source with an ion lens is moved from 0 mm to 2 mm from theright hand side of the aperture in the curtain plate. The improvementobtained with the ion lens becomes more apparent at distances greaterthan 6 mm. At 7 mm, the ion signal for the ionspray source without anion lens, has dropped off to approximately 25% of the maximum ionsignal. However, the ion signal obtained for the ionspray source withthe ion lens is still above 50% of the maximum ion signal. At a distanceof 8 mm, the ion signal for the ionspray source without an ion lens hasdropped off to approximately 1% of the maximum ion signal, whereas theion signal for the ionspray source with the ion lens Is still greaterthan 46% of the maximum ion signal. In fact, an ion signal is maintainedeven at a distance of 14 mm with the ion lens in place. Thus, FIG. 18shows that the dependence of the ion signal on the horizontal positionof the sprayer for the ion source decreases when an ionspray source withan ion lens is used.

Reference is next made to FIG. 19 which shows the dependence of the ionsignal on the vertical position of the sprayer of an ionspray sourcewithout an ion lens (represented by ‘o’ shaped data points) and thesprayer of an ionspray source with an ion lens (represented by ‘+’shaped data points). This data was collected with the sprayer of theionspray source located just off to the right hand side of the aperturein the curtain plate. From FIG. 19, the maximum ion signal for bothionspray sources was at a vertical position of approximately 0 mm (i.e.the sprayer was at the same vertical height as the middle of theaperture in the curtain plate). The experimental data shows that at allpositions higher and lower than the center of the aperture in thecurtain plate, a stronger ion signal was obtained for the ionspraysource with an ion lens. Moving the position of the sprayer of theionspray source without the ion lens 5 mm higher resulted in an ionsignal which was approximately 1% of the maximum ion signal, whereas atthe same position for the ionspray source with the ion lens, the ionsignal was 70% of the maximum ion signal. Further increases in theheight of the sprayer for the ionspray source without the ion lensresulted in complete elimination of the ion signal. However, with theion lens in place, a strong ion signal (35% of the maximum ion signal)was maintained even at a vertical height of 15 mm above the center ofthe aperture in the curtain plate. Similar results were obtained as thesprayers were lowered by up to 5 mm. FIGS. 18 and 19 show that the ionsignal is much less sensitive to position when an ion lens is used, evenwithout optimizing the ion lens potential at each position.

Tables 1-3 and FIGS. 15-19 have shown that the addition of an ion lensto an ionspray source yields a stronger and more stable ion signal.Furthermore, the addition of the ion lens results in an apparatus whichis much easier to operate since the position of the sprayer can vary afew millimeters without having an extremely detrimental effect on theresulting ion signal. Two important factors were the position of the ionlens along the sprayer tip and the potential applied to the ion lens.Favorable results were achieved when the ion lens was located preferably1-3 mm behind the up of the sprayer of the ionspray source. A range ofdifferent ion lens sizes were also found to be useful for the ionspraysource. The increased signal stability and the decreased dependence uponsprayer position for optimization are important benefits, particularlyfor applications such as isotopic analysis, LC mass spectrometry and CEmass spectrometry where the position of the sprayer can have a dramaticeffect on the observed ion signal.

Reference is next made to FIG. 20, which shows that the ion lens resultsin an ion signal which is stable over a wide range of conditions. FIG.20 is a graph of the ion signal on a linear scale versus time, from 0 to16 minutes. The ion signal measured in FIG. 20 was obtained with aProtana reduced flow-rate ion source fitted with an ion lens, whichprovided ions to a Q-Star mass spectrometer made by AppliedBiosystems/MDS Sciex. The applied potentials were 3000 V for thesprayer, 1000 V for the ion lens and 526 V for the curtain plate. Thesprayer had an internal diameter of approximately 15 microns at thetapered end. The sample, a digest of the protein casein, was prepared ina solution containing 90% water and 10% acetonitrile with 1% aceticacid. At approximately 2.8 minutes 170, the potential applied to thesprayer was removed. As a result, the ion signal dropped to zero cps.The potential was then re-applied to the sprayer, at its previous value,at approximately 3.4 minutes 172 and the intensity of the ion signalalso returned to its prior level. At approximately 4.25 minutes 174, thepotential applied to the ion lens was removed and at approximately 4.6minutes 176, the potential was re-applied to the ion lens at itsprevious value. Once again, the intensity of the ion signal dropped tozero cps when the potential applied to the ion lens was removed,however, when the potential was reapplied to the ion lens, the intensityof the ion signal returned to its previous level. The solution flow ratewas then set to zero at 5.13 minutes 178 and then set back to itsprevious value at 5.9 minutes 180. As a result, the ion signal droppedto zero cps when the solution flow rate was zero but then returnedbriefly to its previous level before spiking upwards when the solutionflow rate was set to its previous level. The spike was due to aconcentration effect in the tapered tip of the sprayer due to theevaporation of the solvent. At 7.51 minutes 184, the sprayer was movedback from the curtain plate until the time of 8.13 minutes 188. The ionsignal intensity decreased but was still observed. From the time periodof 8.45 minutes 188 to 12.8 minutes 190, the sprayer was moved to theleft and to the right of the aperture in the curtain plate. Once again,the ion signal was still detectable. For the rest of the test data, theposition of the sprayer relative to the entrance aperture of the massspectrometer was varied in an attempt to eliminate the ion signal. Thesignal remained until the potentials were shut off. The results shown inthe Figure demonstrate that even if the values of certain parameterschange, once the parameters return to their original values, the ionsignal intensity also returns to its original corresponding levels. FIG.20 also demonstrates that this device is effective for samples with ahigh aqueous content (90% aqueous). It is important to note that thedata presented in FIG. 20 is plotted with a linear scale on the y-axis.This causes the ion signal to appear less stable than the data presentedin FIGS. 16 and 17 in which the y-axis has a log scale.

Reference is next made to FIGS. 21 a- 21 d which show the effect of theions lens on charge state over time. The FIGS. 21 a-b are graphs of ionintensity versus time as the lens potential was varied using a Protanaion source. The top panel in FIG. 21 a shows the total ion count for adigest of the protein β-casein as the potential on the ion lens wasincreased from 500 V to 3000 V. The top panel shows that the total ioncount decreased due to a decrease in unwanted singly charged ions whichcontribute to background noise. The second and third panels show thatthere is an increase in the ion signal for triply and doubly chargedpeptide ions with an increase in the potential applied to the ion lens.Therefore, as the doubly and triply charged peptide ion signal increasein intensity, there is a concurrent decrease in the unwanted singlycharged ions that contribute noise. This leads to an increase in thesignal to noise ratio of the ion signal. FIG. 21 b shows an expandedview of the total ion count as the potential applied to the ion lens isincreased. FIGS. 21 c and 21 d show the mass spectrum of the ion signaltaken at 0.43 minutes (point 191 in FIG. 21 b) and 2.1 minutes (point192 in FIG. 21 b). The mass spectrum in FIG. 21 c shows that it isdifficult to detect the triply charged peptide ions at a mass to chargeratio of about 688 (region 193) and the doubly charged peptide ions at amass to charge ratio of about 1031 (region 194). However, the massspectrum in FIG. 21 d, taken when a higher potential was applied to theion lens, shows that the triply charged peptide ion signal 193′ is nowobserved as well as the doubly charged peptide ion signal 194′.Therefore, when a higher potential was applied to the ion lens, theresulting mass spectrum was much less noisy, the ion intensities weregreater, and the signal to noise ratios for the multiply charged peptideions increased.

Reference is next made to FIGS. 22 a and 22 b which show experimentalresults using a reduced flow-rate ion source with and without an ionlens. The sprayer had an internal diameter of 15 μm. FIG. 22 a showsthat singly charged noise ions 198 have a larger presence in the massspectrum than the multiply charged peptide ions 200. The results shownin FIG. 22 a were obtained when the potentials applied to the curtainplate and the sprayer were adjusted to obtain the best ion signalpossible. However, the resulting mass spectrum was still noisy. Incontrast, the mass spectrum in FIG. 22 b shows that, with the additionof an ion lens, much more favorable results can be obtained. Thecontribution of the singly charged noise ions 198′ have been reduced andthe ion signal Intensity for the multiply charged peptide ions 200′ hasincreased from 16 to 44 cps. This represents a signal increase ofapproximately 2.5 to 3 times. This is important for applications inwhich multiply charged ions have to be detected.

Referring now to FIGS. 23 a and 23 b, a sample of glufibrinopeptide wasanalyzed by a mass spectrometer having a standard ionspray source (FIG.23 a) with a flow rate of 3 μL/min and a mass spectrometer having areduced flow-rate sprayer, with a flow rate of 400 nL/min and an ionlens (FIG. 23 b). The Figures show that the ion intensity for a doublycharged ion of glufibrinopeptide 202 was increased from approximately110 cps to 300 cps (peak 204 in FIG. 23 b) with the use of an ion lens.The sensitivity is indicated by the vertical scale on the left of FIGS.23 a and 23 b. This is an increase of about 2.7 times. Furthermore, theuse of the ion lens, resulted in an ion signal with a smaller RSD sincethe ion signal waveform 206 in FIG. 23 b is much flatter than the ionsignal waveform 208. The measured RSD was reduced by a factor of 2 whenthe ion lens was used.

Reference is next made to FIGS. 24 a- 24 d which show the resulting ionsignal for a digest of a 500 fmol sample of beta casein which wasapplied to a reduced flow-rate ion source without and with an ion lens.The flow rate was on the order of 200-400 nL/min. FIGS. 24 a and 24 bshow that the ion lens resulted in an increase in ion signal intensity(212′ versus 212) in the mass spectrum. FIGS. 24 c and 24 d show similarresults in the time domain. With the addition of the ion lens, thebackground noise (214′ versus 214) is decreased and the peptide ionsignal is increased (216′ versus 216). In this case, the signal to noiseratio was increased by a factor greater than 4.

Referring now to FIGS. 25 a and 25 b, the mass spectrum is shown foranother sample of beta-casein digest which was applied to a reducedflow-rate ion source without and with an ion lens, respectively. Theaddition of the ions lens allowed the triply charged peptide peak 218′in FIG. 25 b to be more easily detected whereas without the ion lens inFIG. 25 a, the triply charged peptide peak 218 was difficult to detectdue to its low intensity and the high magnitude of the background noise.The intensity of the peptide peak was increased by a factor of 3.5 timeswith the addition of the ion lens.

Referring now to FIGS. 26 a and 26 b, the graphs show the magnitude ofthe background noise in the vicinity of the triply charged peptide 218and 218′ shown in FIGS. 25 a and 25 b, respectively. FIG. 26 a is thebackground noise for the reduced flow-rate ion source in the absence ofthe ion lens and FIG. 26 b is the background noise with the ion lens.FIGS. 26 a and 26 b demonstrate that the background noise is the samewith and without the lens. Therefore, the signal enhancement shown inFIGS. 25 a and 25 b does not lead to an increase in the background noiseand the signal to noise ratio is thus increased by a factor ofapproximately 3.5 times.

Referring now to FIGS. 27 a and 27 b, the mass spectra are shown for abeta-casein digest sample which was applied to a reduced flow-rate ionsource without and with an ion lens, respectively. In the mass spectrumshown in FIG. 27 a (i.e. no ion lens), the doubly charged peptide ionsignal 222 is difficult to detect. However, in FIG. 27 b (i.e. with theion lens), the doubly charged peptide ion signal 222′ is more easilydetected. Also, the ion signal intensity for the doubly charged peptideion signal 222′ is much larger when the ion lens was used.

Referring now to FIGS. 28 a and 28 b, a 100 fmol sample of bovine serumalbumin digest was applied to a nano-HPLC-MS with an ion lens. Theliquid flow rate for the sprayer was 100-300 nL/min and the sprayer hadan inner diameter of 15 μm. The test results showed that there was asufficient increase in the signal to noise ratio when the ion lens wasused. Tandem mass spectrometry (MS/MS) was carried out on the twostrongest peptide ion signals detected in every scan. The total ioncount for peptide fragments from the strongest peptide ion signal isshown in the third panel of FIG. 28 a. The total ion count for thepeptide fragments of the second strongest peptide ion signal is shown inthe fourth panel of FIG. 28 a. The largest number of peptide ions wereobserved around 14 minutes. The top panel in FIG. 28 b shows the massspectrum obtained at 14.53 minutes of the experiment. The bottom panelin FIG. 28 b shows the fragment ion spectrum for the dominant peptideion signal at a m/z ratio of 480.6. This data is important because theresults shown in FIGS. 28 a and 28 b could not be achieved if the ionlens was not used in the ion source.

Reference is next made to FIG. 29 which shows the ion signal measuredfor a 50 fmol digest of bovine serum albumin which was applied to anano-HPLC-MS with an ion lens. The ion lens is very important becausebefore using the nano-HPLC-MS, water must be pumped through the deviceto condition the column. If an ion lens is not used, the ESI interfacewill not operate because water disrupts the spraying process due to itshigh surface tension. A gradient of water and organic solvent was usedto separate hydrophobic and hydrophilic peptides. The test wasprematurely terminated, but the peptides 230 were detected between 11.5to 17 minutes after the test started. The measured ion signal was thenreferenced to a database to identify the digested protein. The proteinwas correctly identified with a certainty of approximately 300 orders ofmagnitude above that which would occur for a random ion signal (i.e. anoise signal). This test result shows that the detection limit for thepeptide ion signal is substantially lower than the 50 fmol of digestused in the experiment. In addition, this test shows that an ion lensgreatly increases the reliability of a nano-HPLC-MS run.

In an alternate embodiment of the present invention, the ion source mayhave more than one ion lens placed in close proximity to the sprayer.Referring to FIG. 30, results are shown for a simulation which showsequipotential lines for an ion source with two concentric ion lensessurrounding a sprayer. The ion source comprises a sprayer 12, a curtainplate 14, an aperture in the curtain plate 15, an orifice 16, an orificeplate 18, a source housing 20, an inner ion lens 240 and an outer ionlens 242. In this simulation, the applied potentials were 3800 V for thesprayer 12, 1800 V for the curtain plate 14, 190 V for the orifice plate18, 4200 V on the inner ion lens 240 and 6000 V on the outer ion lens242. The results show that the equipotential lines are flat directly infront of the tip of the sprayer 12 which focuses the ions towards theaperture 15 in the curtain plate 14.

Reference is now made to FIG. 31 which illustrates the results of asimulation which shows equipotential lines for the same ion sourceconfiguration shown in FIG. 30 except that the potentials applied to theinner ion lens 240 and the outer ion lens 242 are reversed. Thepotential applied to the inner ion lens 240 is 6000 V and the potentialapplied to the outer ion lens 242 is 4200 V. The resulting equipotentiallines are once again flat directly in front of the tip of the sprayer 12which should focus the ions towards the aperture 15 in the curtain plate14.

Reference is now made to FIG. 32 which illustrates the results of asimulation which shows equipotential lines for the same ion sourceconfiguration shown in FIG. 30 except that the ion lenses 240′ and 242′have been slightly misaligned along the axis of the sprayer 12. Apotential of 4200 V is applied to the sprayer 12, a potential of 5500 Vis applied to the ion lens 242′ and a potential of 3500 V is applied tothe ion lens 240′. The curtain plate 14 is biased at a potential of 1800V, the orifice plate 18 is biased at a potential of 190 V and thehousing 20 is at ground. The simulation results show that theequipotential lines are flat directly in front of the sprayer 12 andperpendicular to the axis of the sprayer 12. Accordingly, thisconfiguration should focus the ions towards the orifice 16 in theorifice plate 18.

Reference is now made to FIG. 33 which illustrates the results ofanother simulation which shows equipotential lines for the same ionsource configuration shown in FIG. 30 except that the ion lenses 240″and 242″ have been substantially misaligned along the axis of thesprayer 12. A potential of 4200 V is applied to the sprayer 12, apotential of 5500 V is applied to the ion lens 240″ and a potential of3500 V is applied to the ion lens 242″. The curtain plate 14 is biasedat a potential of 1800 V, the orifice plate 18 is biased at a potentialof 190 V and the housing 20 is at ground. Once again, the simulationresults show that the equipotential lines are flat directly in front ofthe sprayer 12 and perpendicular to the axis of the sprayer 12.Accordingly, this configuration should focus the ions towards theorifice 16 in the orifice plate 18.

Reference is now made to FIG. 34 which illustrates the results ofanother simulation which shows equipotential lines for the same ionsource configuration shown in FIG. 30 except that the ion lenses 240′″and 242′″ are aligned along the longitudinal axis of the sprayer 12.Note that ion lenses 240′″ and 242′″ do not have to have the samedimensions as may be suggested by FIG. 34. A potential of 4200 V isapplied to the sprayer 12, a potential of 5500 V is applied to the ionlens 242′″ and a potential of 3500 V is applied to the ion lens 240′″.The curtain plate 14 is biased at a potential of 1800 V, the orificeplate 18 is biased at a potential of 190 V and the housing 20 is atground. Once again, the simulation results show that the equipotentiallines are flat directly in front of the sprayer 12 and perpendicular tothe axis of the sprayer 12. This configuration should focus the ionstowards the orifice 16 in the orifice plate 18.

The results shown in FIGS. 30 to 34 illustrate that two ion lenses maybe used with an ion source to focus the generated ions towards anaperture. Alternatively, one may also use more than two ion lenses. Thebasic idea is that the incorporation of more than one ion lens providesan opportunity for further optimization via application of potentials tothe extra ion lens(es) so that the equipotential lines can become morefavorable directly in front of the sprayer which may result in an ionsignal that is further enhanced. The extra ion lens may be orientedconcentrically as shown in FIGS. 30 and 31 or misaligned as shown inFIGS. 32 and 33 or aligned longitudinally along the axis of the sprayeras shown in FIG. 34.

In another embodiment of the present invention, the use of an ion lensmay be extended to ion sources that have multiple sprayers. Referring toFIG. 35, a dual reduced flow-rate electrospray ion source 250 is showncomprising a sprayer mounting bracket 252, a mounting hole 254, aconductive tab 256, an ion lens 258, a first capillary 260 and a secondcapillary 262, a first sprayer 264 and a second sprayer 266, twocapillary butt junctions 268 and 269, a syringe pump 270 and anelectrospray power supply 272. The two sprayers 264 and 266 were pulledfrom fused silica capillaries (150 μm outer diameter and 50 μm internaldiameter) to an internal diameter of approximately 15 μm (although otherdimensions may be used). The ion lens 258 was placed approximately 2 mmbehind the end of the tapered tips of the two sprayers 264 and 266. Theion lens 258 was constructed from stainless steel and was oblong inshape similar to the ion lens shown in FIG. 5 a. The aperture of the ionlens 258 (not shown) had a length of 10.3 mm, a height of 4.6 mm and was1.2 mm thick, although other dimensions could be used. The two sprayers264 and 266 were centered in the ion lens 258. Alternatively, otherconfigurations may be used such as those that were previously shown forthe case of a single ion lens and a single sprayer, i.e. the sprayersmay be asymmetrically oriented along one or both dimensions of the ionlens 258. Furthermore, the sprayers may be different lengths. In use,the two sprayers 264 and 266 are operated at a reduced liquid flow-ratesimultaneously with the ion lens 258 located around the tapered tips ofthe sprayers 264 and 266. The solution flow rates ranged from 0.2 μL/minto 1 μL/min. Alternatively, other solution flow rates may be used. Alsonote that more than two sprayers may be used.

Experiments were conducted comparing the dual reduced flow-rate ionsource 250 with an ion lens 258 versus a single reduced flow-rate ionsource without an ion lens and a dual reduced flow-rate ion sourcewithout an ion lens. The applied potentials for the single and dualreduced flow-rate electrospray sources were 3895 V for the sprayers and1000 V for the curtain plate. For the dual reduced flow-rate ESI ionsource 250 with an ion-lens 258, the applied potentials were 4198 V forthe sprayers 264 and 266, 1840 V for the curtain plate (not shown) and2500 V for the ion lens 258.

The results in Table 4 show the measured ion signal for 10 scans of asample of 10⁻⁵ M bradykinin. Table 4 indicates that doubling the numberof sprayers increased the ion signal by a factor of 1.6. The addition ofthe ion lens further increased the signal Intensity by a factor of 1.34.Therefore, the combination of the extra sprayer and the ion lensresulted in an improvement in the ion signal intensity by a factor of2.2. In theory, to achieve this increase in ion signal intensity withextra sprayers and no ion lens, 5 sprayers would be required.

TABLE 4 Measured ion signal for 10 scans of a Bradykinin sample Dualreduced Single reduced Dual reduced flow-rate flow-rate flow-rateelectrospray Sprayer electrospray electrospray with an ion lens (P +2H)²⁺ signal (cps) 2.05 × 10⁶ 3.28 × 10⁶ 4.45 × 10⁶

Another advantage of the multiple sprayers with the ion lens is thereduced dependence of the strength of the ion signal upon the sprayerposition relative to the aperture in the curtain plate. As more sprayersare positioned in front of the aperture, they become positioned furtherfrom the optimal location, leading to a decrease in the effectiveness ofeach additional sprayer. Thus, the improvement in ion signal intensitywill decrease with the use of more sprayers. However, the use of anionlens positioned around the sprayers should help alleviate this problem.

Referring now to FIG. 36, the results of a simulation performed on adual reduced flow-rate ion source 280 without an ion lens is shown. Thedual sprayer ion source 280 comprises a first sprayer 282, a secondsprayer 284, a curtain plate 286, an aperture 288, an orifice plate 290,an orifice 292 and a housing 294. The applied potentials in thesimulation were 4000 V for the sprayers 282 and 284, 1000 V for thecurtain plate 286, and 190 V for the orifice plate 290. The housing 294was maintained at ground. The resulting equipotentials are curved nearthe tip of the sprayers 282 and 284 which results in a much wider spreadof ion trajectories 296. The defocusing nature of the equipotentialscauses many ions to be directed away from the orifice 292.

Referring now to FIG. 37, the results of a simulation done on a dualreduced flow-rate ion source 280′ with an ion lens 298 shows theresulting equipotential lines. The dual sprayer ion source 280′comprises all of the elements shown in FIG. 36 for the dual sprayer ionsource 280 in addition to an ion lens 298. The applied potentials in thesimulation were 4300 V for the sprayers 282 and 284, 1800 V for thecurtain plate 286, 5220 V for the ion lens 298, 190 V for the orificeplate 290 and 0 V for the housing 294. The equipotentials lines areflattened near the tip of the sprayers 282 and 284. This causes the ionsto be directed straight towards the aperture 288 in the curtain plate286 and then towards the orifice 292.

The dual reduced flow-rate ion source 280′ with the ion lens 298 shownin FIG. 37 can be operated such that the sprayers 282 and 284 are usedin succession. If two different samples are to be analyzed then onesample may be placed in the first sprayer 282 and the second sample maybe placed in the second sprayer 284. The first sprayer 282 is thenoperated to create ions from the first sample which are thensubsequently analyzed by a downstream mass spectrometer. When theanalysis is complete, the first sprayer 282 is turned off by stoppingthe solution flow. The second sprayer 284 is then operated to createions for the second sample which are then subsequently analyzed by thesame mass spectrometer. In addition, separate power supplies can be usedfor each sprayer, allowing a sprayer to be turned off by controlling theelectrospray potential. This system is preferable versus a system with asingle sprayer when more than one sample needs to be analyzed since thesingle sprayer must be changed/cleaned after each sample is analyzed.Alternatively, more than two sprayers may be used. In an alternativeembodiment, multiple different samples may be sprayed simultaneouslyfrom multiple different sprayers inserted into a single ion lens. Thiswould be beneficial for studies involving the infusion of an internalstandard or mass calibrant. A mass calibrant is useful for calibrationof a mass range in devices such as a time of flight mass spectrometerwhereas an internal standard is useful for determining the concentrationof an analyte in an analysis. An internal standard is also helpful indetecting variations in sprayer efficiency.

Based on FIGS. 30 to 37, there are a variety of embodiments for using anion lens or ion lenses with a sprayer or sprayers. There may be onesprayer and one ion lens surrounding the sprayer. Alternatively, theremay be one sprayer and a plurality of ion lens surrounding the sprayer.There may also be a plurality of sprayers and one ion lens thatsurrounds the sprayers.

In the experiments, it has been observed that under some circumstances,the voltage on the ion lens cannot be increased above the voltage on thesprayer since the electrospray ceases and a droplet is observed to growat the tip of the sprayer. This may occur because the electric field atthe tip of the sprayer decreases to the point where the electric fieldis insufficient to overcome the surface tension of the droplet. However,as commonly known to those skilled in the art, a small fraction ofmethanol or other organic solvent may be used in the analyte sample todecrease the surface tension of the forming droplet which may lead toincreases in the maximum potential applied to the ion lens which mayfurther increase the ion signal.

The principles of substantially atmospheric pressure ion lenses weredescribed for ESI, ionspray, reduced flow-rate ionspray, reducedflow-rate ESI and nanospray sources used in conjunction with a massspectrometer. However, the principles of the present invention can alsobe utilized for capillary electrophoresis mass spectrometry,microchannel ESI mass spectrometry and the transfer of ions for otherpurposes such as, but not limited to, ion deposition onto surfaces toproduce coatings. The present invention may also be applied toatmospheric pressure chemical ionization sources where ionization isproduced at a corona discharge tip. The present invention may further beused for depositing a sample in ion sources which employ Matrix AssistedLaser Deposition ionization. The invention may further be used toprovide ions that could be used in downstream regions that are atatmospheric pressure, sub-atmospheric pressure and at or near vacuum.Furthermore, the results shown for reduced flow-rate electrospray ionsources may also correspond to those which may be expected from reducedflow-rate ionspray sources.

It will be readily apparent to those skilled in the art that theinvention can be modified in the number and shape of the ion lensessituated in the vicinity of the capillary tip without departing from thefundamental principles and spirit of the invention.

It will also be apparent to those skilled in the art that: 1) allpotentials used in this description are relative and that for example,the sprayer may be operated at a potential of 0 V with the curtain plateand orifice plate operated at a high negative potential and the ion lensat an intermediate negative potential to produce positive ions; 2) thepresent invention can apply equally to negative ions provided that allof the potentials previously described are reversed in polarity; and, 3)the solution flow rates are not limited to those described herein whichare for illustrative purposes only.

It should be understood that various modifications can be made to thepreferred embodiments described and illustrated herein, withoutdeparting from the present invention, the scope of which is defined inthe appended claims.

1. An ion source apparatus for generating ions from an analyte sample,the apparatus comprising an ion source, at least one counter electrodeand an ion focusing element, wherein the ion source is mounted oppositesaid at least one counter electrode and the ion focusing element ismounted relative to the ion source, whereby, in use, with a potentialdifference applied between the ion source and said at least one counterelectrode to generate a plurality of ions, and to cause the plurality ofions to move towards said at least one counter electrode, and with apotential applied to the ion focusing element to change theequipotentials adjacent the ion source to focus and direct ions in adesired axis of ion propagation.
 2. The apparatus of claim 1, whereinthe ion focusing element is located adjacent to the ion source.
 3. Theapparatus of claim 1, wherein ions are directed along an axis extendingfrom the ion source and wherein the equipotentials adjacent the ionsource are substantially perpendicular to the desired axis of ionpropagation, both on the axis and for a substantial area around theaxis.
 4. The apparatus of claim 1, wherein the ion source, the at leastone counter electrode and the ion focusing element are mounted in ahousing.
 5. The apparatus of claim 4, wherein the housing is one of thecounter electrodes.
 6. The apparatus of claim 4, wherein the interior ofthe housing is at substantially atmospheric pressure.
 7. The apparatusof claim 4, wherein the apparatus includes an orifice plate with aninlet orifice disposed downstream of a curtain plate that has anaperture and closes off the housing, wherein the ion source, the atleast one electrode and the ion focusing element are adapted to directthe generated ions towards the inlet orifice, whereby in use, a greaterand more stable flux of generated ions passes through the inlet orifice.8. The apparatus of claim 4, wherein the apparatus includes an inletplate having an inlet capillary closing off the housing, wherein the ionsource, the at least one electrode and the ion focusing element areadapted to direct the generated ions towards the inlet capillary,whereby in use, a greater and more stable flux of generated ions passesthrough the inlet capillary.
 9. The apparatus of claim 7, wherein theorifice plate is part of an inlet of a mass spectrometer.
 10. Theapparatus of claim 4, wherein the apparatus further comprises at leastone power supply connected to the ion source and the ion focusingelement, connectible in use to the at least one counter electrode, andadapted to provide different DC potentials thereto.
 11. The apparatus ofclaim 2, wherein the ion focusing element comprises an ion lens and anattachment element, wherein the attachment element is adapted to receivea constant potential which is applied to the ion focusing element todirect and focus the generated ions.
 12. The apparatus of claim 11,wherein the ion lens is mounted to surround substantially the tip of theion source.
 13. The apparatus of claim 12, wherein the ion lens isgenerally planar and is placed substantially perpendicular to thelongitudinal axis of the ion source.
 14. The apparatus of claim 12,wherein the ion lens is placed at an angle to the longitudinal axis ofthe ion source.
 15. The apparatus of claim 12, wherein the ion lens isan annular lens having at least one of a continuous and discontinuouscross-section, said cross-section having a shape substantially similarto one of a circle, an oval, a square, a rectangle, a triangle and anyother regular and irregular polygon.
 16. The apparatus of claim 12,wherein the ion lens is placed at or behind the tip of the ion source.17. The apparatus of claim 16, wherein the ion lens is placedapproximately 0.1 to 5 mm behind the tip of the ion source.
 18. Theapparatus of claim 16, wherein the ion lens is placed approximately 1 to3 mm behind the tip of the ion source.
 19. The apparatus of claim 16,wherein the ion lens is placed approximately 2 mm behind the tip of theion source.
 20. The apparatus of claim 12, wherein the ion lens has anaperture and the tip of the ion source is symmetrically located along atleast one dimension of the aperture.
 21. An ion source apparatus forgenerating ions from an analyte sample, the apparatus comprising: an ionsource; at least one counter electrode mounted opposite the ion source;and, an ion lens mounted adjacent to the ion source, wherein, duringuse, a potential difference is applied between the ion source and the atleast one counter electrode to generate a plurality of ions, and tocause the plurality of ions to move towards the at least one counterelectrode, and a constant potential is applied to the ion lens to chancethe equipotentials adjacent to the ion source to stabilize the spray ofionized droplets and to focus and direct the ions in a desired axis ofion propagation.
 22. The apparatus of claim 8, wherein the apparatusfurther includes a curtain gas disposed in front of the inlet capillary.23. The apparatus of claim 22, wherein the inlet capillary is part of aninlet of a mass spectrometer.
 24. The apparatus of claim 22, wherein theapparatus includes a plurality of ion focusing elements which aremounted to substantially surround the tip of the ion source.
 25. Theapparatus of claim 24, wherein the plurality of ion focusing elementsare coaxially mounted in a common plane to substantially surround thetip of the ion source.
 26. The apparatus of claim 25, wherein there aretwo ion focusing elements, the first ion focusing element beingpositioned to surround the tip of the ion source and the second ionfocusing element being coaxially positioned around the first ionfocusing element.
 27. The apparatus of claim 24, wherein the pluralityof ion focusing elements are spaced apart from one another along thelongitudinal axis of the ion source.
 28. The apparatus of claim 12,wherein the ion focusing element is adjustably mounted.
 29. Theapparatus of claim 12, wherein the ion lens has an aperture and the tipof the ion source is asymmetrically located along at least onedimensions of the aperture.
 30. The apparatus of claim 1, wherein theapparatus comprises at least two ion sources and the ion lens ispositioned in close proximity to the at least two ion sources tosurround substantially the at least two ion sources.
 31. The apparatusof claim 30, wherein the ion lens is placed behind the tip of at leastone of the at least two ion sources.
 32. The apparatus of claim 31,wherein the ion lens is placed approximately 0.1 to 5 mm behind the tipof at least one of the at least two ion sources.
 33. The apparatus ofclaim 31, wherein the ion lens is placed approximately 1 to 3 mm behindthe tip of at least one of the at least two ion sources.
 34. Theapparatus of claim 31, wherein the ion lens is placed approximately 2 mmbehind the tip of at least one of the at least two ion sources.
 35. Theapparatus of claim 12, wherein the ion lens has an aperture withadjustable dimensions for further focusing and directing the generatedions.
 36. The apparatus of claim 1, wherein the ion source is at leastone of an atmospheric pressure chemical ionization source, a reducedflow-rate electrospray ion source, a reduced flow-rate ionspray source,an electrospray source, an ionspray source, a nanospray source, and amatrix assisted laser desorption ionization ion source.
 37. A method forgenerating ions from an analyte sample, the method comprising the stepsof: 1) supplying the analyte sample to an ion source; 2) providing atleast one counter electrode spaced from the ion source; 3) providing apotential difference between the ion source and said at least onecounter electrode to generate a plurality of ions; and, 4) providing anion focusing element and applying a potential to the ion focusingelement to change the equipotentials adjacent to the ion source to focusand direct the plurality of ions in a desired axis of ion propagation.38. The method of claim 37, wherein the method further comprisesproviding the ion focusing element adjacent to the ion source.
 39. Themethod of claim 37, wherein the ions are directed along an axisextending from the ion source and wherein the method further comprisesadjusting the potential applied to the ion focusing element to ensurethat the equipotentials adjacent to the ion source are substantiallyperpendicular to the desired axis of ion propagation, both on the axisand for a substantial area around the axis.
 40. The method of claim 38wherein the method further comprises providing at least one power supplyconnected to the ion source and the ion focusing element, connectible inuse to the at least one counter electrode and providing different DCpotentials to the ion source and the ion focusing element.
 41. Themethod of claim 38, wherein the method further comprises providing anion lens and an attachment element, wherein the method further comprisesproviding a constant potential to the attachment element for enablingthe ion focusing element to direct and focus the generated ions.
 42. Themethod of claim 41 wherein the method further comprises mounting the ionlens to surround substantially the tip of the ion source.
 43. The methodof claim 42, wherein the method further comprises mounting the ion lensso that the ion source abuts or intersects a plane defined by the ionlens.
 44. The method of claim 41, wherein the ion lens has an apertureand the method further comprises adjusting the aperture to further focusand direct the generated ions.
 45. The method of claim 41, wherein thereare at least two ion sources, the method further comprises the step ofplacing the ion lens to surround substantially the tip of the at leasttwo ion sources and the ion lens is placed behind the tip of at leastone of the at least two ion sources.
 46. The method of claim 37, whereinthe method further comprises the step of: 5) providing the generatedions to a downstream mass analysis device.
 47. The method of claim 42,wherein the method further comprises the step of; 5) providing thegenerated ions for ion deposition to coat surfaces.
 48. The method ofclaim 46, wherein the method further comprises the steps of: 5) placingsimilar analyte samples in each ion source; and, 6) operating each ionsource simultaneously, whereby, the overall flux of ions generated fromthe analyte sample is increased.
 49. The method of claim 45, wherein themethod further comprises the steps of: 5) placing different analytesamples in each ion source; and, 6) operating each ion sourcesequentially, whereby, switching between the different analyte samplesis facilitated.
 50. The method of claim 45, wherein the method furthercomprises the steps of: 5) placing an analyte sample in one ion sourceand a mass calibrant in another ion source; 6) operating each ion sourcesimultaneously; and, 7) passing the generated ions into a mass analyzerfor mass analysis, whereby, the mass calibrant is used to calibrate themass analyzer.
 51. The method of claim 45, wherein the method furthercomprises the steps of: 5) placing an analyte sample in one ion sourceand an internal standard in another ion source; 6) operating each ionsource simultaneously; and, 7) passing the generated ions into a massanalyzer for mass analysis, whereby, the internal standard is used toassess ion source efficiency and aid in analyte quantitation.
 52. Themethod of claim 45, wherein the method further comprises the steps of:5) placing an analyte sample in one ion source and a different analytesample in another ion source; 6) operating each ion sourcesimultaneously; and, 7) passing the generated ions into a mass analyzerfor mass analysis.
 53. The method of claim 37, wherein the methodfurther comprises optimally positioning the ion source and applyingappropriate potentials to the ion focusing element and the ion sourcesuch that the magnitude of the ion signal derived from the generatedions is increased.
 54. The method of claim 37, wherein the methodfurther comprises optimally positioning the ion source and applying anappropriate potential to the ion focusing element and the ion sourcesuch that the relative standard deviation of an ion signal derived fromthe generated ions is decreased.
 55. The method of claim 37, wherein themethod further comprises optimally positioning the ion source andapplying an appropriate potential to the ion focusing element such thatthe charge states of the generated ions is changed.
 56. The method ofclaim 37, wherein the method further comprises optimally positioning theion source and applying an appropriate potential to the ion focusingelement such that the ion fragmentation of an ion signal derived fromthe generated ions is changed.
 57. The method of claim 37, wherein themethod further comprises optimally positioning the ion source andapplying an appropriate potential to the ion focusing element such thatthe intensity of unwanted background noise ions is reduced.
 58. Themethod of claim 37, wherein the method further comprises applying anappropriate potential to the ion focusing element such that the ionsource and the ion focusing element can be used in a broader range ofpositions relative to a downstream orifice or capillary.
 59. Theapparatus of claim 7, wherein the apparatus further includes a curtaingas emanating from between the orifice plate and the curtain plate. 60.An ion source apparatus for generating ions from an analyte sample, theapparatus comprising: an ion source: at least one counter electrodemounted opposite the ion source; and, a plurality of ion focusingelements mounted to substantially surround the tip of the ion source,wherein, in use, a potential difference is applied between the ionsource and the at least one counter electrode to generate a plurality ofions, and to cause the plurality of ions to move towards the at leastone counter electrode, and potentials are applied to the plurality ofion focusing elements to change the equipotentials adjacent to the ionsource to stabilize the spray of ionized droplets and to focus anddirect the ions in a desired axis of ion propagation.
 61. The apparatusof claim 60, wherein the plurality of ion focusing elements arecoaxially mounted in a common plane to substantially surround the tip ofthe ion source.
 62. The apparatus of claim 60, wherein the plurality ofion focusing elements are spaced apart from one another along thelongitudinal axis of the ion source.